1 35-ounce can San Marzano tomatoes
5 tablespoons extra virgin olive oil, or to taste
1 medium onion, (about 2 cups), sliced thin
4 slices pancetta, cut into 1 1/2-inch julienne strips
½ teaspoon crushed red pepper flakes
1 pound perciatelli or bucatini pasta
1 cup Pecorino Romano, grated, plus more for passing
Pass the tomatoes and their liquid through a food mill fitted with the fine disc. Set aside. Bring 6 quarts of salted water to boil in an 8-quart pot.
In a large skillet, heat 2 tablespoons of the olive oil over medium heat. Add the onions and cook, stirring, until wilted, about 4 minutes. Stir in the pancetta and cook 2 minutes. Add the hot red peppers and the strained tomatoes and bring to a boil. Adjust the heat to a simmer, and season lightly with salt. Cook, stirring occasionally, until the sauce is thickened, about 20 minutes.
Meanwhile, stir the perciatelli into the boiling water and cook, stirring occasionally, until done, about 12 minutes.
Check the seasoning of the sauce, adding salt if necessary (remember the Pecorino is mildly salty).
Reserve about 1 cup of the pasta cooking water. Drain the pasta, return it to the pot and pour in half the sauce. Bring the sauce and pasta to a boil and add enough of the pasta cooking water, if necessary to make enough of the sauce to lightly coat the pasta. Check the seasoning, adding salt if necessary. Remove the pan from the heat, stir in 1 cup of the grated cheese and transfer to a large, heated serving platter or bowl. Spoon the remaining sauce over the top and pass additional grated cheese separately if you like.