Cut the pancetta or guanciale into 1/2 inch strips or small cubes.
In a large skillet over medium heat, sauté the pancetta in the olive oil. After a minute or two, add the onion. Cook over low heat until the onion is golden.
Bring a large pot of salted water to a boil.
Cook the pasta until al dente (about 2 minutes less than package directions). Drain, and retain about one cup of cooking liquid. Add pasta to the pancetta and onion, mix well, and cook over medium heat for minute. Add some of the retained water if it looks dry.
Place the pasta in a serving bowl, add the Pecorino and a generous amount of black pepper. Mix well.
In Rome, guanciale is often used, but I prefer the meatier pancetta.