British watercress is one of the great joys of the spring kitchen. Grown in the chalk streams of Hampshire and Dorset, it arrives at the shops with a proper peppery bite and a deep, glossy green that no imported bag can match. Blitz it with sweet peas and a spoonful of crème fraîche and you have a soup that tastes like the first warm afternoon of the year: light, vibrant, ready in about half an hour.

Why watercress belongs in a soup

Watercress is a brassica, a cousin of mustard and rocket, which is where that lovely heat comes from. The stems are the most peppery part, so please do not throw them away — they give the soup its backbone. The leaves are softer and sweeter, and they need barely any cooking at all.

Paired with peas, the flavour opens up. Frozen peas are harvested and frozen within hours, so in spring they are often sweeter and more reliable than the fresh ones in a supermarket chiller. Add a starchy potato for body, use a good light stock rather than a heavy cube, and finish with crème fraîche for a gentle tang. Nothing else is needed. This is a soup that rewards restraint.

What you will need

This makes four generous bowls or six starter portions.

• 25g unsalted butter, plus a little olive oil to stop it catching
• 2 banana shallots, finely sliced, or 1 small mild onion
• 1 small leek, white and pale green parts, washed and sliced
• 250g floury potatoes, peeled and cut into 2cm cubes — Maris Piper or King Edward
• 900ml light chicken or vegetable stock, hot
• 200g frozen peas
• 150g watercress, roughly two bags, stalks and leaves separated
• 4 tbsp crème fraîche, plus extra to serve
• A squeeze of lemon juice, sea salt, white pepper and a grating of nutmeg

How to make it, step by step

• Melt the butter with a splash of oil in a large pan over a low heat. Add the shallots and leek with a good pinch of salt. Sweat gently for 8–10 minutes until completely soft and translucent. Do not let them colour — any browning dulls the finished green.
• Add the potatoes, stir to coat, then pour in the hot stock. Bring to a gentle simmer and cook for 12–15 minutes, until the potato crumbles easily against the side of the pan.
• Tip in the frozen peas and the watercress stalks. Simmer for 2 minutes, no longer.
• Add the watercress leaves, push them under the surface, and count to thirty. Take the pan straight off the hob.
• Blend until completely smooth. A jug blender gives the silkiest result; if you use a stick blender, keep the head under the surface to avoid splattering, and blend for a full minute. For a truly refined texture, pass the soup through a fine sieve with the back of a ladle.
• Stir in the crème fraîche off the heat, then season with salt, white pepper, a little nutmeg and a squeeze of lemon. Taste again — the lemon is what makes the peas sing.
• Serve warm rather than scalding, with an extra swirl of crème fraîche thinned with a drop of milk, a few raw watercress leaves and a grind of black pepper.

The one kitchen skill that matters here

Green soups are lost or won in the last two minutes. Chlorophyll — the pigment that makes watercress so vivid — breaks down quickly with heat, and it does so fastest in the presence of acid. So keep the cooking time after the greens go in to an absolute minimum, take the pan off the heat before blending, and only add the lemon once the soup is off the hob.

If you want restaurant-level colour, blanch the watercress leaves in boiling water for 20 seconds, lift them straight into iced water, drain, and blend them into the finished soup at the very end. It is one extra bowl to wash, and it makes a noticeable difference. A high-speed blender also helps: the finer the purée, the brighter the colour reads.

Variations for the rest of spring

• Wild garlic: in April and May, swap a handful of the watercress leaves for wild garlic. Go carefully — it is far more pungent than it looks.
• Broad beans: from June, use 200g podded and skinned broad beans in place of the peas for a nuttier, more substantial bowl.
• Herbs: a few sprigs of mint, chervil or lovage blitzed in with the watercress add a lovely top note. Mint in particular loves peas.
• Lighter still: drop the potato, use a good vegetable stock, and rely on the crème fraîche for body. It becomes a delicate broth-like soup — lovely as a starter, less filling as a lunch.
• More substantial: top with a poached egg, a few shards of grilled ham or a scatter of toasted pumpkin seeds.

Serving, storing and using up

Serve this in warm shallow bowls with thick slices of buttered sourdough or a cheese scone. It is a natural first course, but with bread and a handful of dressed salad it makes a perfectly good light lunch. Add a glass of something crisp and dry if the sun is out.

Keep leftovers in the fridge for up to three days. Reheat very gently — a bare simmer, never a boil — and expect the colour to fade a little each time; a fresh squeeze of lemon and a new spoonful of crème fraîche will bring it back. The soup freezes well for up to three months, but freeze it before adding the crème fraîche and stir that in once reheated. Portion it in bags laid flat so it thaws in minutes on a busy weekday.