Carefully clean the closed mussels with a brush under running water. Put them in a saucepan, add a little water and let them boil until the mussels open. Separate the mussels that have not opened! Filter the broth, remove the mussels from their shells, rinse well repeatedly and drain. Preheat the oven to 180 degrees.
Melt butter in a frying pan and sauté the tomatoes. Season with salt and pepper, add the mussel stock and cook. Add vinegar and sugar and when the sauce is creamy, remove from the heat. Pour the tomato sauce into a buttered baking dish, add the mussels and cook in the oven for 15 minutes. Sprinkle with cheese and bake again for 10 minutes.
Serve in the form together with fresh white bread.