Kulajda: Czech Dill and Mushroom Soup With a Poached Egg

There is a moment, about four minutes into eating kulajda, when the poached egg gives way and the yolk runs out into the pale green soup around it. The soup goes from sour and sharp to sour and rich in a single stir. That is the whole design of the dish, and it is why the egg sits on top instead of being beaten in somewhere earlier: the yolk is a sauce you deploy yourself, at the table, at the moment you want it.

Kulajda is a soup from southern Bohemia — the Šumava forests, the pond country around Třeboň — and it belongs to a family of thickened sour soups that runs right across central and eastern Europe. Its close relatives include the Polish żurek, which gets its acidity from fermented rye water rather than vinegar, and the Romanian ciorbă de perișoare, soured with borș. Every one of them solves the same problem: how do you make a soup out of almost nothing — water, roots, whatever the forest gave you — and make it taste like something you want to eat twice a week for six months of the year? The answer is acid, dairy and a lot of one herb.

Where kulajda comes from, and why it tastes so strange the first time

The word itself is a puzzle. The most persistent etymology traces it to the German Kaltschale, a cold dish, which is odd for a soup that everyone now serves piping hot. Others link it to a dialect verb for stirring or mixing. Czech cookbooks from the nineteenth century record thickened dill soups under several names, and kulajda in something close to its current form is really a twentieth-century stabilisation of an older peasant idea: potatoes and soured cream and whatever mushrooms came out of the pine woods after rain.

South Bohemia matters here. The region is criss-crossed with artificial carp ponds dug in the fifteenth and sixteenth centuries, and the land between them is sandy pine forest — which is exactly where boletes grow. A household there would have had potatoes after they arrived in the Czech lands in the eighteenth century, soured cream from the family cow, dill in the garden bed, and free mushrooms from September onwards. Kulajda is that inventory turned into lunch.

The first spoonful confuses most people who did not grow up with it. It is creamy in a way that suggests a mild soup, and then the vinegar arrives and the dill arrives and it is aggressively green and sharp. The trick is to stop expecting it to behave like a cream of mushroom soup. It is closer in spirit to a hot, thick, vegetal version of soured cream itself. Once your palate accepts the sourness as the point rather than a mistake, the whole thing snaps into focus.

Mushroom picking in the Czech Republic is a genuine national pastime — houbaření — with an intensity that surprises visitors. People take the first train out on an autumn Saturday, know their family’s spots to within a few metres, and refuse to tell you where they are. A properly made kulajda in September uses whatever came home in the basket that morning. The rest of the year it uses dried porcini, which is what I have specified here, because a good dried bolete rehydrated in hot water gives you more concentrated mushroom flavour than most fresh mushrooms available in a British supermarket.

The mechanics: why the cream does not split

The single technical hurdle in kulajda is keeping the soured cream smooth. Soured cream is around 18–20% fat with a lot of casein and a pH already on the acidic side. Drop it straight into a boiling, acidic, salty pan and the proteins seize into visible grains — the soup tastes fine and looks like curdled wallpaper paste.

Three things prevent it, and you need all three.

Flour first. Whisk the plain flour into the cold soured cream before anything hot touches it. The starch granules sit between the casein micelles and physically get in the way of them clumping. This is why the traditional method is a jíška of sorts — the flour is doing structural work, and the slight thickening is a bonus.

Temper properly. Ladle a good 250 ml of the hot soup into the cream mixture while whisking, and only then return it to the pan. Going from fridge-cold to 95°C in one step is what breaks it. Going in two steps, with agitation, gets the cream up to temperature gently enough that the proteins stay dispersed.

Never boil it afterwards. Once the cream is in, hold the soup at a bare simmer, with the occasional lazy bubble breaking at the edge and nothing more. Five minutes at that heat cooks out the raw flour flavour without pushing the dairy over the edge. If you walk away and it boils hard, you will see the surface go slightly grainy. It is still edible; it is no longer beautiful.

Room-temperature cream helps too. Take it out of the fridge when you start peeling potatoes.

The acid balance, which is the actual recipe

Everything else in kulajda is assembly. The sourness is where the cooking happens.

Soured cream brings its own lactic tang, but it is nowhere near enough on its own. You need added vinegar, and the amount depends entirely on your cream: a supermarket soured cream is much milder than a proper crème fraîche or a Czech smetana. Start at 2 tablespoons of white wine vinegar for this quantity, taste, and go up in teaspoons. The finished soup should make you notice the sourness on the sides of your tongue without wincing.

The teaspoon of sugar is doing something specific. It does not sweeten the soup — at that dose you will never taste it as sweetness. It softens the leading edge of the vinegar so the acid reads as roundness instead of a spike. Leave it out and the soup tastes thin and pointed. This is the same principle behind the pinch of sugar in a good vinaigrette.

Caraway is the other Czech signature, and it goes in early with the potatoes so its oils have time to leach into the water. A teaspoon is enough to add a faint aniseed-adjacent warmth underneath everything without announcing itself. If you dislike caraway, halve it rather than dropping it — the soup misses its floor without something in that register.

Dill goes in at the very end, off the heat or nearly so. Dill’s flavour compounds are volatile and heat-sensitive; boil it for ten minutes and you get grassy sludge with none of the perfume. Thirty grams sounds like an enormous quantity because it is, and it should be. Kulajda without an assertive dill presence is just potato soup with an identity crisis.

What goes wrong

Watery, thin soup. Usually too little flour, or waxy potatoes that gave up no starch. Kulajda should coat the back of a spoon lightly. If yours is loose, mash a few of the potato cubes against the side of the pan with a spoon and stir them back in.

Gluey, wallpaper soup. The opposite: floury potatoes disintegrating plus the flour in the cream. Use waxy potatoes — Charlotte, Anya, anything labelled salad potato — and cut them at 1.5 cm so they hold their shape through 14 minutes of simmering.

Grey, sad mushrooms. You crowded the pan and steamed them. Chestnut mushrooms hold a lot of water; give them a hot pan and enough space, wait for the liquid to boil off, and only then let them brown. That browning is worth real flavour.

Flat, boring soup. Almost always under-vinegared. Taste it again and be braver.

Egg poached to a rubber ball. Three minutes in trembling water, no more. Very fresh eggs hold together; older ones spread into rags. If yours are a week old, crack each into a small sieve first and let the loose outer white drain away before poaching.

Choosing the mushrooms

Kulajda tolerates a wide range of fungi and rewards a narrow one. The gold standard is the bay bolete or the penny bun — Boletus edulis, the porcini — because their flavour survives both drying and a long soak in cream. Chanterelles work and bring an apricot note that suits the dill surprisingly well, though they are expensive enough that using them here feels like showing off.

Field mushrooms and chestnut mushrooms are the practical everyday choice, and they need the browning step to earn their place. A raw-tasting agaricus simmered in cream contributes moisture and very little else. Give them a genuinely hot pan and let them take colour, and you are extracting the same browned compounds that make a mushroom on toast worth eating.

Avoid anything that leaks black — inky caps, some older boletes — because the soup’s pale green is half its appeal. Shiitake are a bad fit: their flavour is too directional and fights the dill rather than sitting under it. Dried porcini are sold in most supermarkets now, and 15 g is a small handful. Soak them in water that has come off the boil and stopped bubbling, never in water still boiling, which cooks the outside before the centre softens and leaves you chewing leather.

The soaking liquid is the most valuable thing in this recipe and the most commonly thrown away. It carries the glutamates that make the finished soup taste savoury underneath its sourness. Pour it in, stop before the sandy last tablespoon, and you have added depth that no amount of extra cream would buy.

Variations, and the case against my version

Some south Bohemian cooks skip the mushrooms entirely — the oldest versions are dill, potato and cream, full stop, with mushrooms as a seasonal luxury. That soup is cleaner and greener, and worth trying once.

A serious minority replaces the poached egg with a hard-boiled one, sliced. This is the version you get in a lot of pubs, and I think it is a downgrade: the whole point is the running yolk enriching the bowl. Some households add a splash of milk to lighten the cream, which suits people who find the full-fat version heavy.

And here is my honest reservation about my own recipe: I have specified dried porcini, which is not what anyone in Třeboň would do in October. If you can get fresh boletes, use 250 g of them and skip the dried entirely, using 200 ml extra water. The dish loses a certain woody depth in the dried version and gains a certain concentrated intensity — that trade is real, and if you have the fresh mushrooms, take them.

Serve kulajda on its own in a deep bowl with dark bread. It is a first course in Czech restaurants, but at home it is dinner, and something like bramborák alongside would be redundant carbohydrate on carbohydrate. If you want to build a proper Czech meal around it, follow with svíčková — though be warned that two cream sauces in one sitting is a commitment.

Leftovers keep three days in the fridge. Reheat gently, never to a boil, and poach a fresh egg each time. The soup thickens overnight; loosen it with a splash of water and check the vinegar again, because it always tastes flatter on day two.