3 tbsp Kosher salt
1 pound spaghetti
1/4 cup extra virgin olive oil
1/2 cup freshly grated Parmigiano-Reggiano, plus extra for serving
4 ounces sliced guanciale or pancetta, or good American bacon, cut into1/2-inch-wide strips
1/2 cup grated pecorino romano
1 medium red onion, halved lengthwise, ends trimmed, and cut lengthwise into 1/4-inch-wide slices
1/3 cup coarsely chopped fresh Italian parsley
1 tablespoon coarsely ground black pepper
Bring 6 quarts of water to a boil in a large pot and add 3 tablespoons kosher salt.
Meanwhile, combine the oil, guanciale, and onion in another large pot and cook over medium-high heat, stirring frequently, until the guanciale is lightly browned and the onion is softened, about 7 minutes. Stir in the pepper and remove from the heat.
Drop the pasta into the boiling water and cook until just al dente. Drain, reserving about 1/2 cup of the pasta water.
Add the pasta and 1/4 cup of the reserved pasta water to the guanciale and stir and toss over medium heat until the pasta is well coated (add a splash or two more of the reserved pasta water if necessary to loosen the sauce).
Stir in the cheeses and parsley and serve immediately, with additional grated Parmigiano on the side.