Clean the mussels and wash them thoroughly. Carefully tap open mussels onto a work surface or similar. If they do not close or move, throw them away. Peel and halve the onions and cut into strips. Peel and slice the garlic
Heat 3 tablespoons of oil in a pot, fry onions and garlic until translucent. Season with salt and pepper. Add white wine and 750 ml water, bring to the boil. Add the mussels, cover and cook over medium heat for about 8 minutes
Pour the mussels onto a sieve and collect the stock. Throw away unopened mussels
Cook the pasta in boiling salted water according to the instructions on the packet. Remove the sausage from the skin and cut into slices. Remove mussel flesh from the skin, except for 10-12 pieces. Grate cheese
Brush a pan with 1 teaspoon of oil. Fry the slices of sausage for 3-4 minutes while turning, add the tomatoes, butter, tomato paste and 300 ml mussel stock, simmer for 4-5 minutes, fold in the whole mussels, pasta and cheese. Season pasta with salt, pepper and a little lemon juice