· 3 large bunches watercress, destalked and chopped – about 8 oz (225 g) de-stalked weight (reserve a few leaves for garnishing)
· 4 oz (110 g) butter
· the white parts of 5 leeks, about 14 oz (400 g), washed and chopped
· 4 medium potatoes, about 1 lb 8 oz (700 g) in weight, peeled and chopped
· 3 pints (1.75 liters) 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 thick-based saucepan, then add the prepared leeks, potato and watercress and stir them around so that they're 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 about 10-15 minutes or until the vegetables are quite tender. Then remove the pan from the heat and when it's cooled a little liquidise the soup – you'll need to do this in batches.
Then return it all to the saucepan, swirl in three tablespoons of crème fraîche, season to taste and reheat very gently. Then serve in hot soup bowls and garnish each one with a little extra crème fraîche and some watercress leaves.