Clean the mussels and put them to boil with the onion and black pepper until they open up. Cut the mushrooms into small pieces and cook them until soft with 1-2 tablespoons of butter. Remove the mussels from their shells and separately fry the flour with 2-3 tablespoons of butter.
Add the cream and a little of the broth from the mussels to get a nice sauce. Pour in the wine, a little black pepper and lemon, cut into slices. Once the sauce comes to a boil, put in the mussels with stewed mushrooms. Simmer the dish for 10-15 minutes, then serve.