Tomato Kasundi: Bengali Mustard Relish

If mango chutney is the gentle, sweet ambassador of the chutney family, kasundi is its wild, mustard-fuelled relative from Bengal — pungent, tangy, fiercely spiced, and completely addictive once you get the taste for it. In its most traditional form, kasundi refers to a fermented mustard paste, a Bengali condiment so revered it was historically prepared with near-ritual care. The tomato version, thick and jammy and shot through with ground mustard, is the one that travelled: Bengali cooks made it, Australian delis of the twentieth century adopted and spread it, and now it turns up on cheese boards and in sandwiches far from where it began. It is the most useful jar in my fridge.

That near-ritual care was no exaggeration. By tradition the fermented kasundi of Bengal was prepared in spring, around the month of Boishakh, and hedged with strict rules of cleanliness: the ground mustard was set to cure and ferment in the fierce sun, handled only by those considered ritually clean, a measure of how highly mustard’s pungency was prized in a cuisine that leans on it as oil, paste and heat. The tomato kasundi here is the relaxed, keep-in-a-jar descendant of that tradition, built squarely around the mustard.

What sets kasundi apart from every other tomato relish is the mustard. This is not a background whisper of the stuff; it is the leading character, giving the relish a sinus-clearing pungency and a deep, bitter-sharp complexity that plays against the sweetness of the tomatoes and sugar. If you like the mustard heat in a good piccalilli or the fierce edge of an English mustard, kasundi will feel like home. It belongs on the same shelf as a jar of mango chutney, and I reach for it far more often.

The mustard, and the chemistry that makes it fierce

Understanding how mustard heat works is the key to a great kasundi. Whole mustard seeds are almost flavourless. The pungency is created only when the seeds are crushed and mixed with a cold liquid, which triggers an enzyme reaction that produces the volatile compounds responsible for that nose-tingling burn. This is exactly why I grind the seeds and mix them with cold vinegar, then leave the paste to sit for fifteen minutes before it goes near the heat. That rest lets the reaction develop the mustard’s full pungency.

The temperature of the liquid matters enormously. Cold water or vinegar produces a sharp, hot mustard; hot liquid, or cooking the seeds too early, denatures the enzyme and gives a milder, more bitter result. Using cold vinegar and adding the developed paste later in the cooking preserves as much of that characteristic heat as possible. I use a mix of yellow mustard seeds for volume and mellow warmth and brown seeds for extra pungency, ground coarsely so a little grittiness remains in the finished relish, which is part of its character.

The clever twist: roast the tomatoes first

Most kasundi recipes start with raw chopped tomatoes cooked down in the pan. Roasting them first is the step that transforms the relish. Halve the tomatoes, roast them cut-side up in a hot oven until they collapse and their edges char and caramelise, then blend them into a rough purée before they go into the pot. Roasting drives off water and concentrates the tomato flavour, so the relish is deeper and needs far less time on the hob to thicken. The light charring adds a smoky, jammy sweetness that raw tomatoes simply cannot give, and it means you are not standing over a pan for two hours boiling off liquid.

It is a small change in method with an outsized effect. A raw-tomato kasundi tastes bright and sharp; a roasted-tomato one tastes rich, rounded and almost sun-dried underneath the mustard heat. The concentrated fruit stands up to the assertive spicing far better, so nothing tastes thin. Once you have made it this way, going back to the boiled version feels like a waste of good tomatoes.

Building the layers

Kasundi is a relish of layered spice, and the order of operations builds those layers. First the whole seeds — cumin, fennel and nigella — are toasted dry to wake their oils, then bloomed in hot oil to release their fat-soluble aromatics. Mustard oil is traditional here and worth seeking out for its own sharp, horseradish-like pungency, though a neutral oil works if you cannot find it; if using mustard oil, heat it until it just smokes and then let it cool a moment to mellow its raw bite. Into the fragrant oil goes the ginger-garlic paste, turmeric and chilli, fried briefly so the spices cook out and stop tasting raw.

Only then do the roasted tomatoes, sugar, salt and the developed mustard paste join the pan. From there it is a patient simmer, stirring often as it thickens, until the relish turns glossy and dark and the oil begins to bead at the edges — the classic Indian sign that a masala is properly cooked and the spices have released their flavour into the fat. That oil separation is what you are watching for; it tells you the water has gone and the relish will keep.

Heat, balance and getting it right

Kasundi should be a balancing act of four forces: the heat of mustard and chilli, the sweetness of sugar and roasted tomato, the sharpness of vinegar, and the savoury depth of the spices and salt. Taste as it finishes and adjust. If the mustard’s bitterness dominates, a little more sugar rounds it; if it tastes flat, more salt or a splash of vinegar wakes it up. The chilli is yours to control — one teaspoon gives a warm background heat, two makes a relish that means business. Do not skimp on the vinegar and sugar, which together with the salt and oil are what preserve the relish for months.

A word on strength: kasundi is meant to be intense. Used by the spoonful rather than the ladleful, its job is to punch up whatever it sits beside, so resist the urge to tame it into blandness. The pungency mellows a little with age, but a good kasundi always has a fierce edge.

Jarring, keeping and eating

Sterilising the jars properly is what lets kasundi keep for months. Wash the jars and lids in hot soapy water, rinse well, then stand the jars upright in a 120C oven for fifteen minutes until bone dry; boil the lids for five minutes or run everything through a hot dishwasher cycle. Fill the jars while both they and the relish are hot, so the glass does not crack and the cooling relish pulls the lid into a seal. Leave about a centimetre of headspace and wipe every rim clean before sealing, since a smear of relish on the rim stops a clean seal and is exactly where mould takes hold.

Spoon the hot relish into warm sterilised jars, seal at once, and let them cool. Stored in a cool dark place it keeps for up to a year unopened; once opened, keep it refrigerated and it will last a couple of months, the surface layer of oil helping to protect it. The vinegar, sugar and salt preserve the relish, so do not cut the vinegar back to soften the sharpness; that acidity is part of what keeps the jar safe. If a stored jar grows mould on top, a rim was left dirty or the relish went in cold; if it fizzes or the lid domes, it was under-cooked and still holding water, so eat it from the fridge rather than the cupboard. Leave it a week before opening the first jar so the flavours marry and the raw mustard edge settles into the whole.

Then use it on everything. It is magnificent with cheese, especially a mature cheddar or a slab of grilled halloumi, and it lifts a cheese toastie into something worth making properly. Spread it in a bacon or sausage sandwich, spoon it alongside cold meats and pork pies, stir it into scrambled eggs, or dab it onto a burger where it does the work of both ketchup and mustard with far more interest. It is superb with grilled oily fish like mackerel, and a spoonful stirred into a pan of lentils or a simple dal gives an instant depth. Like every good preserve, it turns quick, plain food into something that tastes considered, which is the whole reason to keep a shelf of jars in the first place.