Wash fish thoroughly and pat dry. Peel onion and cut into quarters. Boil plenty of salted water with onion quarters, white wine, lemon juice, up to 1 tablespoon, and bay leaf. Add the fish and let it simmer at medium heat for about 20 minutes. Peel the carrots, leaving a little green, and wash them.
Clean, wash and cut the zucchini into longish strips. Cook the carrots in boiling salted water for about 15 minutes. Add the zucchini strips 5 minutes before the end of the cooking time and cook with them. For the sauce melt 20 g fat, sweat flour in it and deglaze with broth. Bring to the boil while stirring, stir in sour cream. Flavour with horseradish, lemon juice, salt and pepper. Remove cress from the bed and stir into the sauce. Remove fish from the stock and drain. Drain the vegetables and toss in the remaining hot fat.
Bring to the boil while stirring, stir in sour cream. Flavour with horseradish, lemon juice, salt and pepper. Remove cress from the bed and stir into the sauce. Remove fish from the stock and drain. Drain the vegetables and toss in the remaining hot fat. Serve with parsley potatoes