Bring 3-4 l salted water (1 teaspoon salt per litre) to the boil. Peel and chop the onions. Press sausage meat out of the skin. Heat the oil in a large pan, fry the sausage meat coarsely while turning it and take it out. Sauté onions in frying fat. Add tomatoes and boil down for about 10 minutes, stirring frequently.
Meanwhile cook the noodles in boiling salted water according to the instructions on the packet until al dente. Preheat oven (electric cooker: 220 °C/circulating air: 200 °C/gas: see manufacturer).
Wash basil, shake dry and pluck leaves. Grate the Parmesan finely. Pluck mozzarella into small pieces.
Drain the pasta, let it drain briefly and put it back into the pot. Season tomato sauce with salt and pepper. Mix with sausage meat and basil into the noodles. Pour everything into an ovenproof dish. Sprinkle with parmesan and mozzarella. Bake in a hot oven for about 15 minutes.