Peel and finely chop the onions. Peel the potatoes and cut into cubes (2 x 2 cm).
Melt the butter in a saucepan and sauté the onion cubes until translucent. Add the potatoes, fry briefly and then pour in the stock. Simmer for about 30 minutes until the potatoes are soft.
Pluck the cress.
Stir in the sour cream and finely puree the soup. Gradually add the cress and continue to puree. Season to taste with salt, pepper and lemon juice.
Serve sprinkled with a little cress or chopped parsley.
Refine the cress soup with crunchy croutons.