Clean and wash the mussels, peel and quarter the onion. Bring the mussels, onion, bay leaf, wine, salt and pepper to the boil and cook for about 5 minutes until the mussels open. Drain the mussels, collect the stock and sort out the closed mussels.
For the tomato sauce, peel and finely dice the onion and sauté in hot oil. Chop the tomatoes in the tin and add them to the onions with the mussel stock. Let it boil down in an open pot for about 30 minutes.
Season to taste with salt, sugar and cayenne pepper. For the filling, coarsely crumble the toast bread. Peel garlic and dice finely. Wash parsley, dab dry and chop. Grate cheese. Mix bread, garlic, parsley, half of the cheese and eggs.
Season with salt and pepper. Wash, clean and seed the tomatoes and dice the pulp. Break the mussels apart, throw away empty shell halves. Fill shell halves with the bread-herb mixture.
Pour the tomato sauce into a flat casserole dish. Place filled mussels on top. Add tomato cubes and sprinkle with remaining cheese. Bake in the preheated oven (electric: 200 °C/ gas: level 3) for about 15 minutes.