In a large skillet, heat the olive oil over medium heat. Add the pancetta and cook until browned and the fat is rendered, about 5 minutes. Use a slotted spoon to transfer to a paper towel-lined plate.
Add the onions to the skillet and saute until translucent, 7 to 8 minutes. Stir in the garlic and a pinch of crushed red pepper (a generous pinch if you like spicy). Cook, stirring, about 2 minutes. Add the tomatoes and use a wooden spoon to break up any large pieces. Add the pancetta back to the skillet. Bring to a low simmer, cover and cook until the flavors have melded and the sauce is slightly thickened, 20 to 30 minutes.
While the sauce cooks, bring a large pot of water to a boil. Season generously with salt. Cook the pasta according to the package directions. Stir 1/2 cup of the pasta water into the sauce (skip this step if using gluten-free pasta), then drain the pasta. Add the pasta and cheese to the sauce and toss to coat. Serve with additional cheese.