Closely adapted from Oretta Zanini de Vita and Maureen B. Fant. I am sure many of you will be tempted to make this dish with pancetta or bacon--and if you make it that way and like it, more power to you (maybe don't tell any Italians you know!). But for me the guanciale is the key. It's very distinctive (black) peppery cured pork belly flavor is what elevates this dish from just being a basic summer pasta tossed with fresh tomatoes.
Bring your salted water in a large pot to a boil and then turn the heat off. This way it will not take long to boil when you are ready for it.
I found it easiest to make this in a nonstick skillet, although obviously generations of Italian housewives made do just fine with steel. Place the oil and guanciale in a large, deep skillet over medium heat.
Let the guanciale heat gently and begin to render fat and brown. Right when it is becoming crisp, add the onion (if using) with a pinch of salt (if you are using canned tomato, check the ingredients--if they are salted, only add a tiny pinch).
Sauté the onion, stirring occasionally, until it is translucent. Add the tomatoes and chile pepper flakes. Taste a cooked onion--if it is not salty (the guanciale can vary in saltiness) and if you are using raw tomatoes add another pinch of salt. For canned, check the label to see if they have salt added.
Bring the pasta water back to a boil and add your pasta to cook. When it is done, be sure to reserve the cooking water.
Reduce the heat on your onions and tomatoes to low, to simmer gently for 20 minutes. The sauce should thicken a little and the fat should separate to the surface of the mixture.
When the pasta is cooked, drain it and place it in a bowl and toss it with the finely grated pecorino. When the sauce is ready, toss the pasta into it. If it seems to dry and needs to loosen, add a little pasta water. This is not a dish that should need a lot of the pasta water, so add it by the 1/4-cup.
Serve the pasta immediately topped with a few coarse grates of pecorino.