Serves 8
9 oz (250 g) watercress, destalked and chopped, a few leaves reserved for garnishing
4 oz (110 g) butter
the white parts of 5 leeks (weighing about 14 oz/400 g), cleaned and roughly chopped
4 medium potatoes (weighing about 1 lb 8 oz/700 g), peeled and roughly chopped
3 pints (1.75 litres) hot stock made with Marigold Swiss vegetable bouillon powder
4 heaped tablespoons crème fraîche
salt and freshly milled black pepper

First of all, melt the butter in a large, heavy-based saucepan, then add the leeks, potato and watercress and stir them around so they are coated with the melted butter.

Next, sprinkle in some salt, then cover with a lid and let the vegetables sweat over a very gentle heat for about 20 minutes, giving the mixture a good stir about halfway through.

After that, add the stock, bring everything up to simmering point and simmer, covered, for 10-15 minutes, or until the vegetables are quite tender. Then remove the pan from the heat and when the soup has cooled a little, liquidise it in batches, then return it to the saucepan.

Now stir in 3 tablespoons of crème fraîche, season to taste and re-heat the soup very gently, without letting it boil. Serve it in hot bowls and garnish each one with the reserved watercress leaves and an extra swirl of crème fraîche. Alternatively, cool the soup and serve chilled, garnished in the same way.