1. Fill a large (6 qt.) pot with salted water and bring to a boil.
2. Melt the butter in a large frying pan, add bacon and onion, saute over low heat until soft. Turn burner off. Cover a plate with a paper towel (to soak up the grease) and place the bacon strips on it to cool. Cut the bacon into bitesize pieces and return to the frying pan, but leave the heat off.
3. Mix the heavy cream, grated cheese, egg yolk, salt, pepper and parsley in a small bowl, and set aside.
4. When the water comes to a boil, add the linguine and cook for about ten minutes or until al dente, then drain.
5. Slowly mix pasta into the pan with the bacon and onions and heat over a low flame.
6. Pour the heavy cream mixture into the pan and cover all ingredients with it. Continue heating over low flame, constantly stirring until done (about five minutes or so). Serve immediately.