How to Make It
Bring a large pot of salted water to a boil. Add the spaghetti and cook until al dente; drain, reserving 1 cup of the cooking liquid.
Meanwhile, in a large deep skillet, melt the butter. Add the pancetta and cook over moderately high heat, stirring a few times, until lightly browned, about 5 minutes. Add the onions, season with salt and pepper and cook over moderate heat, stirring frequently, until softened, about 10 minutes.
Add the spaghetti to the skillet along with the parsley and the reserved pasta cooking liquid. Cook over moderately high heat, stirring frequently, until the pasta is suspended in a thick creamy sauce, about 2 minutes. Stir in the cheese, season generously with pepper and serve right away.