Peel and dice the potatoes. Finely chop the onion, roast in the melted butter, pour the chicken soup on it, add the potato cubes and simmer covered for 20 minutes. Add two thirds of the cress, let it boil briefly and puree.
Add wormwood and whipped cream, bring to the boil again and season with lemon juice, salt and pepper. Finely chop the rest of the cress, add to the soup, bring to the boil briefly and serve immediately.