In a large, deep skillet, cook the guanciale or pancetta over moderate heat, stirring frequently, until lightly browned, about 6 minutes. Using a slotted spoon, transfer to a plate.
Pour off all but 2 tablespoons of the fat in the skillet. Add the onion, garlic and crushed red pepper and cook over moderately high heat, stirring occasionally, until the onion is lightly browned, about 6 minutes. Return the meat to the skillet. Add the tomato sauce, season with salt and simmer until very thick, about 10 minutes.
Meanwhile, in a pot of salted boiling water, cook the pasta until al dente. Drain the pasta, reserving ½ cup of the cooking water.
Add the pasta to the sauce along with the reserved cooking water and stir over moderately high heat until the pasta is evenly coated, 2 minutes. Split the pasta in 4 pasta bowls, finish with parsley and grated pecorino cheese.