Take the sausage meat out of the casing and form small balls out of it. Quarter the onion lengthways and cut into very fine strips. Also cut the fennel, freed from the stems and stalk, into fine strips. Finely chop the garlic and chop the tomatoes a little.
Fry the bratwurst pieces vigorously on all sides in olive oil. Then add the onions until they are translucent. Now add the chopped garlic and fennel. Salt and pepper and add a pinch of sugar. Now add the tomato sauce, possibly a few sprigs of fresh thyme and the bay leaves. Let the sauce simmer uncovered over low heat for about 15 minutes while cooking the pasta until al dente.