Wash the scallops and pat dry. Fry them briefly in butter until they are light brown. Take out the mussels.
Sauté the onion with the remaining butter and deglaze with sherry (white wine is also possible). Add the cream, fish stock and dill and season with salt and pepper. Add the mussels again and simmer for 2-4 minutes (no longer, otherwise they will be tough!).