Watercress Soup

250 g watercress, de-stalked and chopped, a few leaves reserved for garnishing, 110g butter, the white part of 5 leeks (weighing about 400 g), cleaned and roughly chopped, 4 potatoes (about 700 g), peeled and roughly chopped, 1.75 litres hot stock made with Marigold Swiss vegetable bouillon powder, 4 heaped tablespoons creme fraiche, salt and freshly milled black pepper

First of all, melt the butter in a large, heavy-based saucepan, then add the chopped leeks, potatoes and watercress and stir them around so they are coated in 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, blend it in batches, then return it to the saucepan.
Now stir in 3 tablespoons of creme fraiche, 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 creme fraiche.