Cook the guanciale in a splash of oil until well browned. Do not drain the oil. Add the tomatoes and red pepper flakes and bring to a simmer. Drop the pasta into well salted boiling water and cook 6 or 7 minutes until it has just the slightest crunch left to it. Remove from the water and add straight to the sauce. Add a few ladles of the pasta cooking liquid along with the cheese. Stir well. Taste for seasoning (guanciale and the cheese will vary in saltiness so you might need to add more salt) Serve.