Why tom yum belongs in your midweek rotation

Most of us keep a handful of dishes that deliver far more than the effort they demand, and this is one of them. Tom yum — the hot and sour soup you will have met in Thai restaurants, often brought to the table in a chafing dish over a flame — is essentially an infusion. You simmer a small handful of aromatics in water or stock for ten minutes, add prawns and mushrooms at the last moment, then finish the pot off the heat with lime juice and fish sauce. From cupboard to table in well under half an hour, and most of that is simply waiting for the water to come to a simmer.

The version below is the clear, brothy sort rather than the creamy tom yum goong nam khon you sometimes see thickened with evaporated milk. It is sharper, lighter and, I think, better suited to a British kitchen on a grey Tuesday. It is also forgiving: once you understand what each ingredient is doing, you can swap and adjust without fear.

The four aromatics that do the heavy lifting

If you take one thing away from this, let it be this: the character of tom yum comes from four ingredients, and none of them are the prawns. They are lemongrass, galangal, makrut lime leaves and bird's eye chillies.

• Lemongrass — two fat stalks. Trim away the dry top two-thirds and the woody base, then bruise the remaining pale stem firmly with the back of a knife or a rolling pin and cut it into short lengths. Bruising ruptures the cells and releases the citrus oil; slicing alone does not do the job half so well.
• Galangal — a thumb-sized piece, about five centimetres, sliced into coins. It looks like ginger and behaves nothing like it: piney, sharp and faintly medicinal. If you cannot find it, ginger will do, but the soup will taste rounder and less perfumed.
• Makrut lime leaves — six or so, torn rather than chopped, which is the traditional way and releases more oil. Frozen leaves are perfectly good and often better than tired fresh ones.
• Bird's eye chillies — two or three, bruised with the flat of a knife. Add them whole if you want heat without a mouthful of fire, or slice them if you do.

All four are sold in most Asian grocers and increasingly in supermarket herb sections. They freeze well, so buy a bundle and keep the surplus in a bag in the freezer for next time.

Prawns, mushrooms and the order of things

For two generous bowls you want around 250–300g of raw peeled prawns. Raw matters: cooked prawns turn to rubber the moment they meet hot liquid. If yours are frozen, defrost them thoroughly and pat them dry. Shells, if you have them, are worth ten minutes in the pot with the aromatics — a quick prawn stock is far better than a stock cube.

Mushrooms are the traditional companion. Straw mushrooms from a tin are the authentic choice and, honestly, the right one; their firm, slightly slippery texture holds up beautifully in the broth. Otherwise use oyster mushrooms torn into strips, or quartered chestnut mushrooms, which are less traditional but work perfectly well. Add them before the prawns, because they need a few minutes to soften and give up their savour.

Building the broth

Bring one litre of water or light chicken stock to a gentle simmer. Add the lemongrass, galangal, lime leaves and chillies. Simmer, uncovered, for ten minutes — long enough for the liquid to taste distinctly of all four, but not so long that it turns bitter. Taste it: it should already smell like the finished dish.

Add the mushrooms and simmer for three or four minutes. Then slide in the prawns and poach them gently for no more than two minutes, until they curl and turn opaque. Now take the pan off the heat. This is the step people skip, and it is the one that matters most.

Off the heat, stir in two tablespoons of fish sauce, the juice of two limes — three if your limes are small and mean — a small pinch of sugar and, if you have it, a teaspoon of nam prik pao, the roasted chilli paste that adds a faintly smoky depth. Taste again. Scatter over coriander and sliced spring onion and serve straight away.

Balancing hot, sour, salty and sweet

Thai cooking is largely a matter of balancing these four, and tom yum lets you practise on a small scale. If the broth tastes flat, it usually needs salt, which means fish sauce, not table salt. If it tastes heavy or dull, it needs lime. If it tastes harshly sour, a pinch of sugar will round it off. Heat comes from the chillies and the chilli paste, and you can always add more at the table.

Do taste as you go and trust your own palate over any recipe, including this one. The quantities above are a starting point, not a rule.

Serving and variations

Serve it in deep bowls with steamed jasmine rice alongside, or on its own as a starter. A few notes: lime juice loses its brightness quickly, so add it at the end and do not reheat leftovers hard. The aromatics are not meant to be eaten — leave the galangal and lemongrass in the bowl and push them to one side as you go.

Variations worth trying: swap prawns for squid or firm white fish; add halved cherry tomatoes for sweetness; use a splash of coconut milk for a richer, southern-style bowl. Once the base is second nature, you can improvise freely, and that is exactly when a recipe has done its job.