Peel the potatoes and onions, cut into small cubes and fry in the butter for a few minutes. Then fill up with broth and cook for 20 minutes. Let cool a little (shouldn`t boil anymore), add the cream and horseradish and puree.
Place the fish and the sliced cress on the filled plates.
The taste becomes more intense if you add the fish in the pot. Instead of trout fillets, you can also use smoked salmon, or both.