Simple red-sauce rigatoni with onion, tomato, olive oil, butter, basil, and parmesan. Use a smaller pan when cooking a half batch of sauce; 1 cup onion is totally fine.

Boil rigatoni in salted water until al dente and reserve some pasta water.
In a pan over medium heat, cook onion in olive oil and butter until soft and sweet, then add tomatoes (and optional tomato paste). Simmer until glossy and concentrated. If making half sauce, use a smaller pan so the sauce reduces properly. Add salt and pepper to taste.
Toss pasta with the sauce, basil, and parmesan, loosening with a splash of pasta water if needed. Serve hot with extra parmesan.