Cut the pancetta into fine strips. Take the tomatoes out of the tin, cut them into pieces and strain the pulp through a sieve. Peel the garlic and cut into fine slices.
Bring enough salted water to the boil in a large pot (without oil) and cook the pasta in it according to the instructions on the packet until al dente.
In the meantime, finish the sauce: melt the butter in a pan. Briefly sauté the onions, garlic and the whole peperoncini in it. Stir in the tomatoes and let simmer briefly over a low heat. Salt and pepper.
Drain the pasta, mix with the sauce and fish out the peperoncini.
Serve the penne sprinkled with parmesan and parsley.