Peel onions and garlic. Finely dice garlic and 1 onion. Cut 2 onions into slices. Heat the fat in a pot. Fry diced onion and about 1/3 of the garlic in it until transparent.
Add rice and sauté briefly. Add 350 ml wine and 200 ml stock, bring to the boil, cover and cook over a low heat for about 20 minutes. When the liquid is absorbed, add 450 ml stock gradually, stirring from time to time.
Clean the mussels, wash them thoroughly and sort them out. Heat oil in a pot, fry onion slices and the remaining garlic until transparent. Season with salt and pepper. Add 400 ml white wine and 300 ml water and bring to the boil.
Add the mussels and cook in a closed pot at moderate heat for about 8 minutes. Stir occasionally. Wash parsley, dab dry, remove coarse stalks, chop leaves roughly and add about 2/3 to the mussels.
Pour mussels onto a sieve. Collect approx. 100 ml stock. Discard unopened mussels. Wash the tomatoes, dab dry, fold into the risotto and simmer for 2-3 minutes. Season to taste with salt and pepper.
Parmesan shavings. Arrange risotto and mussels on preheated plates, sprinkle some stock on each mussel. Sprinkle with the remaining parsley and parmesan.