In a 12 to 14-inch saute pan, combine guanciale, garlic, onion and chili flakes and saute until onion is soft. Season with salt and pepper. Add tomato sauce, reduce heat and allow to simmer 10 to 15 minutes.
Cook bucatini according to package directions, until tender but still al dente, about 12 minutes. Remove pasta from heat, drain and add to simmering sauce. Toss to coat and divide among 4 heated bowls. Serve immediately, topped with freshly grated pecorino.