Cullen Skink: Smoked Haddock and Potato

Cullen skink is proof that a soup with four main ingredients and a slightly alarming name can be one of the finest things Britain makes. Smoked haddock, potatoes, onions or leeks, and milk: that is very nearly the whole list, and yet done well it is deep, silky, sweet with allium and heavy with clean woodsmoke, the kind of bowl that makes you go quiet for a minute. Done badly it is watery, over-salted and grey. The gap between the two comes down to a handful of decisions, and every one of them is easy once you know why it matters.

A soup named for a place and a beast

The name repays a moment’s attention. Cullen is a small fishing town on the Moray Firth in the northeast of Scotland, a coast that lived on the herring and haddock trade for centuries. Skink is the older, odder word: it comes from a Scots and ultimately Middle Dutch term for a shin or knuckle of beef, and by extension the soup or broth made from it. So a skink was originally a meat broth, and cullen skink is the fishing-town adaptation, the soup a household made when the beef ran short but there was always smoked fish drying in the smokehouse. A poverty dish, in other words, of exactly the sort that so often turns out to be a masterpiece.

The fish is where Cullen’s history lives. The Moray coast smoked haddock over hardwood as a means of preservation long before refrigeration, and the finnan haddie — split, brined and cold-smoked haddock, named for the nearby village of Findon — is the aristocrat of the family and the traditional fish for the soup. If you can find a proper finnan haddie or an Arbroath smokie, use it. If not, the rule that matters is this: buy undyed smoked haddock. The bright canary-yellow fillets are dyed to imitate the colour real oak smoke used to give, and they carry a harsh, one-note smokiness. Undyed haddock, pale straw-coloured and gently smoked, gives a rounder, cleaner flavour and lets you taste the fish underneath.

The one technique everything hinges on

If you take a single thing from this recipe, take this: you steep the haddock in the milk, you do not boil it. Set the fillets in a wide pan, cover with milk, bring it only to the first shiver of a simmer, then cut the heat, cover the pan and walk away for eight to ten minutes. The fish cooks gently in the retained heat until it flakes, staying moist and tender, and the milk quietly draws out the smoke and the fish’s own savour, becoming the flavoured backbone of the whole soup. Boil the fish hard and you get two problems at once: rubbery, stringy haddock, and a milk that can catch or curdle. Steeping solves both, and it is nothing more than a matter of turning the heat off at the right moment.

That poaching milk is liquid gold, so you strain it and keep every drop. It goes back into the soup at the end, off a hard boil, carrying all the smoke it absorbed. This is why cullen skink tastes so profoundly of the fish even though the fish itself is a relatively small part of each bowl.

Bringing it together, gently

With the potatoes done, pour in the reserved smoky milk and bring the whole thing to a bare warmth. A spoonful or two of double cream is optional and takes it towards the richer, restaurant version; I leave it out as often as I put it in, because good milk and crushed potato already give plenty of silk. Finally, fold in the flaked haddock and let it warm through for two minutes only. Do not let it boil. Everything past this point is about temperature restraint: keep it below a simmer and the fish stays tender and the milk stays smooth.

Season last and taste before you reach for salt. Smoked haddock is salty, sometimes very, and it is entirely possible to build a cullen skink that needs no added salt at all. Black pepper, on the other hand, it wants generously. Chives and parsley go in at the end for freshness and colour against all that pale, smoky cream.

What can go wrong, and make-ahead notes

The classic disasters are all avoidable. A split, grainy milk means it boiled — keep it gentle from the moment the milk returns to the pot. A thin, watery soup usually means waxy potatoes that refused to break down, or none of them crushed; fix it by mashing a few more against the side. An overwhelming, acrid smokiness almost always traces back to dyed haddock, so start with the right fish. And an over-salted bowl comes from seasoning before tasting the naturally salty fish.

Cullen skink keeps for two days in the fridge and reheats well over low heat, though the fish softens further, so some cooks hold a little flaked haddock back to add fresh. It does not freeze happily; the potato goes grainy and the milk can split on thawing. Serve it with plenty of buttered bread or oatcakes, in bowls warmed first so the delicate soup does not lose its heat too fast.

If you love the pairing of smoke and gentle poaching, kedgeree with smoked haddock and curried butter uses the very same fish and the same steeping trick for the flakes, and makes a natural next dish once you have a smokehouse habit. For more soups where the potato itself does the thickening and no flour comes near the pot, Scotch broth with barley and lamb shows the same faith in a single humble ingredient to carry a whole bowl. Make cullen skink once with undyed fish and a gentle hand on the heat, and you will understand why a fishing town’s thrift became a national treasure.