1. Heat oil and butter in a large skillet over medium heat. Add guanciale and cook, stirring, until barely beginning to brown. Add onion and garlic and cook, stirring, until translucent.

2. Crush tomatoes and add to skillet. Cook, crushing tomatoes with the back of a spoon, until sauce has thickened, about 15 minutes; season with red pepper flakes and salt.

3. Bring a large pot of salted water to a boil, add bucatini and cook until al dente, about 5 minutes. Drain and transfer to skillet. Return skillet to heat and fold pasta and sauce together until heated through, about 5 minutes. Remove from heat. Fold in 1 1/2 cups cheese; season with salt and serve immediately with more cheese, if desired.