In a skillet large enough to hold the cooked pasta, add the oil, pancetta, and onion. Cook, stirring, over medium heat until the pancetta and onion are golden, about 12 minutes. Add the garlic the last 5 minutes.
Push the onions and pancetta off to one side and add the can of tomatoes and crushed red pepper. With a potato masher, crush the tomatoes into small chunks. If you don’t have a potato masher just crush tomatoes with your hands into the pan. Stir well. Add salt to taste. Bring to a simmer and cook until thickened, about 20 minutes. Stir occasionally.
Bring 5 quarts of water to a boil and add a tablespoon of salt. Cook the pasta till it’s al dente.
Save about 1 cup of the pasta water. Drain the pasta and add to the pan with the sauce. Toss on high heat until the pasta is coated for about a minute. Add some of the pasta water if it’s too dry. Remove from the heat and add the Pecorino Romano cheese and toss some more.