Wash the mussels thoroughly, brushing them off if necessary. Sort out already opened mussels and do not use them any further. Peel and finely chop the onions. Peel garlic and also dice finely.
Wash parsley, dab dry and chop finely. Cut off bread crust. Grate bread finely. Mix 5 tablespoons olive oil, bread, parsley, salt and pepper. Heat 3 tablespoons of olive oil in a pan, sauté half of the onions and half of the garlic in it, add tomato paste.
Deglaze with chunky tomatoes, stir in ketchup and season to taste with salt, pepper, sugar and paprika. Let it boil down at low heat for about 5 minutes. Grate the Parmesan finely. Heat 2 tablespoons of olive oil in a large casserole dish, sauté the remaining onions and garlic.
Add mussels, deglaze with wine and 1/2 litre of water. Cover and simmer at medium heat for 12-15 minutes. Drain the mussels. Discard mussels that are still closed. Remove the empty mussel halves from the opened ones.
Place the mussel halves on a baking tray with the mussel meat facing upwards. Fill half of the mussels with parsley, the rest with tomato sauce. Sprinkle with parmesan and sprinkle with the remaining olive oil.
Bake in the preheated oven (electric oven: 225 ° C/ gas: level 4) for 8-10 minutes. Serve mussels garnished with parsley and lemon slices. Baguette tastes good with it.