Cut the pancetta into a small dice and add to a sauté pan. Cook this over medium heat.
Cut the onion into a small dice and add to the pan with the pancetta. The fat from the pancetta will cook the onion.
Bring a pot of salted water to a boil. Add in the bucatini and cook until al dente, about 9 minutes. (Be sure to reserve 1/2 a cup of pasta water.)
Beat the egg in small bowl.
Add the pasta to the sauté pan with the egg, pasta water, and cheese.
Slowly sauté all of the ingredients together over low heat until the sauce becomes thick, about 30 seconds. (Please note that the egg should not be scrambled.)