3/4 ounce parmigiano-reggiano cheese, finely grated (plus more for serving)
freshly ground black pepper
1 lb dried spaghetti pasta
1 tablespoon unsalted butter
Heat the olive oil in a large (straight-sided, preferably) skillet over medium heat. Add the diced pancetta and cook, stirring frequently, until most of the fat has rendered into the pan and the pancetta is crispy and caramelized slightly. Using a slotted spoon, remove the crispy pancetta and place in a seperate small bowl, leaving any fat behind in the pan.
Over medium-low heat, add the diced onion and saute in the rendered fat for 3 to 5 minutes, stirring frequently, until the onions are soft and lightly caramelized. Add the garlic and continue to saute for an addition 2 to 3 minutes, being careful to avoid burning the garlic or onion. Deglaze the pan with a tablespoon of water, if necessary, to remove any bits of crispy pancetta or bits from the bottom of the pan.
Add the tomato paste and red pepper flakes, and continue to cook, stirring frequently, over medium heat for an addition 1 to 2 minutes, or until the paste has thickened slightly.
Add the pancetta, and any extra rendered fat, back into the skillet, along with the crushed tomatoes. Bring the sauce to a low simmer, and cook, stirring every few minutes, for 15 to 20 minutes. The sauce should be relatively thick. Add the grated cheese to the sauce, and season to taste with salt and pepper. Place over low heat while you cook the pasta.
Meanwhile, bring a large pot of salted water to boil. Add the pasta and cook until al dente. Reserve 1/2 to 3/4 cup of the pasta cooking water in a measuring cup, and then drain the pasta. Return the drained pasta back into the pot, toss with a tablespoon of unsalted butter, and add the sauce, thinning with pasta cooking water as necessary until the pasta is evenly coated. Serve pasta with extra grated parmigiano-reggiano cheese on the side.