In a large skillet over medium heat, sauté the onion in the olive oil for 2 minutes. Add the pancetta and the Brussels sprouts. Over medium low heat, cook together until the ingredients are golden. Add salt and pepper to taste.
Bring a large pot of salted water to a boil.
Cook the pasta until al dente (about 1–2 minutes less than manufacturer’s directions) drain, and reserve 1 cup of the cooking water. Add the pasta and some of its water to the Brussels sprouts mixture and cook for one minute.