In a large, deep skillet, cook the pancetta over moderate heat, stirring frequently, until lightly browned, about 6 minutes. Using a slotted spoon, transfer the pancetta 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 pancetta 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 1/2 cup of the cooking water.

Add the pasta to the sauce along with the parsley and the reserved cooking water and stir over moderately high heat until the pasta is evenly coated, 2 minutes. Serve the pasta in bowls, passing the cheese at the table.