1/4 cup butter
1 leek, thinly sliced
1/2 pound potatoes, peeled and thinly sliced
2-1/2 cups chicken stock
Pinch grated nutmeg
Salt and freshly ground black pepper
4 large bunches of watercress, cleaned and trimmed, (reserve a few sprigs for garnish)
4 teaspoons cream
Melt the butter in a large saucepan and cook the leek gently until soft. Stir frequently to prevent browning. Add the potatoes, stock, nutmeg and seasonings. Bring to a boil, then cover and simmer 15 minutes. Add watercress and simmer an additional 10 minutes.
Cool the soup slightly, then process in a food processor or blender until the vegetables are finely chopped. Rinse the saucepan, then strain the soup through a fine sieve into the pan. Push all but the tough stalks through the sieve with a wooden spoon.
Season to taste, add the cream, and heat gently -- do not boil. Serve garnished with watercress sprigs and cream.