In a large skillet, over medium heat, cook the guanciale in the olive oil for a minute. Add the onions. When the onion and pancetta are golden, add the tomatoes, salt and pepper – and if using, crushed red pepper. Cook for about 30 minutes, or until the sauce has reduced.
Cook the bucatini until al dente (about 2 minutes less than the package directions), drain, and add it to the skillet with the sauce. Cook together for one minute.