Wash potatoes, cook in boiling water for 15-20 minutes, peel. Meanwhile wash fish fillets. Bring 3/4 litre salted water and bay leaf to the boil in a wide pot, add lemon juice. Let the fish fillets simmer for about 5 minutes at low heat. Stir the contents of the bag of crab soup into 250 ml of lukewarm water. Add 200 ml of the fish stock and cook for 3 minutes at low heat while stirring. Wash the dill, parsley and prawns.
Chop the herbs. Stir dill and prawns into the soup. Refine with crème fraîche. Melt the butter and toss the potatoes and parsley in it. Arrange fillets, potatoes and sauce in portions. Serve garnished with lemon. Serve with a green salad