To make the rich, brown sauce that is key to the flavor of this dish, it is important not to rush the initial searing of the meat. To make peeling the pearl onions easier, soak them first in warm water for 15 minutes.
Use pancetta or the traditional choice for this dish, guanciale. The meat gives this simple sauce a rich flavor. If you buy the meat at an Italian market, have the butcher slice it for you, and then cut it into strips when you get home.