2 T extra-virgin olive oil
6 oz. pancetta (1/4-inch-thick sliced) or bacon, cut into strips about 1 inch long and 1/4 inch wide
1 medium onion , chopped fine
1/2 tsp red pepper flakes or to taste
2 1/2 cups diced tomatoes with juice
1 pound bucatini , perciatelli, or linguine
1/3 cup grated Pecorino Romano cheese (about 1 1/2 ounces)
Bring 4 quarts water to rolling boil in large stockpot or Dutch oven.
Meanwhile, heat oil in large skillet over medium heat until shimmering, but not smoking. Add pancetta or bacon and cook, stirring occasionally, until lightly browned and crisp, about 8 minutes. Transfer pancetta or bacon with slotted spoon to paper towel–lined plate; set aside. If necessary, drain all but 2 tablespoons fat from skillet. Add onion to skillet; sauté over medium heat until softened, about 5 minutes. Add red pepper flakes and cook to release flavor, about 30 seconds. Stir in tomatoes and salt to taste; simmer until slightly thickened, about 10 minutes.
While sauce is simmering, add 11/2 teaspoons salt and pasta to boiling water. Cook until pasta is al dente; drain and return pasta to empty pot.
Add pancetta to tomato sauce and adjust seasoning with salt. Add sauce to pot with pasta and toss over low heat to combine, about 30 seconds. Add cheese and toss again; serve.