Prepare covering sauce using butter, flour and milk. Easy microwave method is to first melt butter, add diced onion and microwave on Hi for 1 min. Stir in flour and add milk. Microwave at Hi until mixture starts to boil and thicken. Stir occasionally to prevent lumping. Otherwise, use basic white sauce technique and saute onions in fry pan, adding later. Add parmesan cheese and salt and stir. In a separate bowl, beat egg yolks slightly and add a small amount of the sauce to the eggs. Stir and then pour into the sauce, stirring constantly. Set aside. Brown the ground beef until cooked and drain off the fat. Add to sauce (any prepared sauce is fine, slightly spiced works best) and set aside. Note: Any homemade spagetti sauce works here, and if it contains ground beef then omit the above addition of meat. Slice the mozzarella cheese. Grease a 9"x13"x2"deep pan. Add a layer of spagetti sauce on the bottom. Add a single layer of noodles. Add a layer of mozzerella, and a layer of ricotta (or cottage cheese). Repeat layers until all ingredients are used up, retaining enough noodles to top it off. Pour (white/cheese) sauce over the lasagna and spread it evenly. Sprinkle some parmesan cheese over the top. Bake at 325F for 25 minutes. Broil to brown the top. Let stand for 10 minutes before serving. Serve with French bread and garlic butter.