Onions peel and in fine cubes cut. Melt butter and fry onions in it. Add 1 1/2 litre of water, bring to the boil and stir in the soup paste. Simmer for 1-2 minutes. Wash dill, shake dry and chop finely.
Stir the crème fraîche into the soup. Season to taste with salt, pepper, lemon juice and sherry. Warm the prawns and mussel meat briefly in the soup. Sprinkle with dill.