To make the cheese topping, sauté onion in butter. Add flour and blend in milk; cook until as thick as heavy cream. Add Parmesan cheese. To a little of the hot cheese mixture, stir in egg yolks, then return to remaining mixture, stirring quickly. (This may be made ahead of time. Cover with Saran Wrap to prevent crusting.) Make 2 layers of sauce and noodles, then layer mozzarella cheese, then 1 or 2 layers of noodles and sauce. Pour cheese topping over to cover. Bake at 325° for 25 minutes. Broil a few minutes to brown top if desired.