Bring a large pot of salted water to a boil and salt the water.
Heat a large deep over medium high heat. Add oil and pancetta. Cook pancetta 2 or 3 minutes then add onions and garlic and crushed red pepper flakes. Cook 7 or 8 minutes more, until onions are translucent. Add tomatoes and parsley. Season the sauce with salt and pepper. Simmer sauce over low heat until ready to serve.
Cook bucatini pasta to al dente or, with a bite to it. Drain pasta well. Do not rinse. Starchy pasta holds more sauce. Toss hot pasta with sauce and serve. Pass grated cheese at the table.
Pancetta is Italian rolled, cured pork, similar to bacon, but not smoked. Look for it at your deli counter. Bacon may be substituted resulting in a smoky tomato sauce.