Boil water: Bring a large pot of salted water to boil for the pasta.
Prep the guanciale: Use a sharp knife to trim the thick bottom skin of the guanciale (discard or reserve it to flavor a soup). Cut the guanciale into slices about 1/8 inch thick and then into 3/4- by 1/4-inch strips (or into cubes).
Heat the olive oil: In a large saute' pan, add the olive oil. When it is shimmering, add the guanciale and the onions. Cook until the guanciale gets crispy (about 10-15 minutes).
Add the wine: As soon as the guanciale is crispy and renders its fat, add the wine and deglaze the pan. Let it simmer together on a medium-high heat for a few minutes.
Add in tomatoes: Next, add in the tomatoes. Use your hands to break up the tomatoes (I like to remove the hard stem). Use a wooden spoon to stir and keep breaking the tomatoes. You could also use tomato passata. Lower the heat and let it simmer gently, stirring occasionally, until the sauce is dense, 20 to 30 minutes.
Season with salt: If using chili pepper, go ahead and add a little. Add the parsley and a little salt. Start with a little bit of salt, as the guanciale is salty. Let the sauce cook and simmer for 20-25 minutes. If it's too thick, ladle in a little bit of the pasta water.
Cook the pasta: When the sauce has about 10 minutes of time to simmer, cook the pasta. Set your timer to 2 minutes before the cook time.
Simmer the sauce: While the sauce is simmering, if it seems too thick, ladle in a little bit of the pasta water.
Drain the pasta: Drain the pasta a couple minutes before it is al dente (reserve a ½ cup of pasta water)and add the pasta immediately to the pan with the sauce.
Combine pasta and sauce: Turn the heat to medium-high and stir to combine with the sauce and coat the pasta. Add in a little bit of the pasta water to loosen the sauce if it’s too thick. Let it simmer together for a minute (you want it to be al dente, not mushy), and plate up the pasta.
Plate up the pasta: Serve with some Pecorino Romano cheese. Enjoy!