Cook manicotti acording to package. Run cool water over it to cool quickly

In a large skillet cook onion and mushroom with the olive oil. Add ground beef when liquid from the mushrooms has evaporated. Drain and set aside.

In a sauce pan, melt butter, whisk in flour until incorporated then stir in cream slowly. (3 cups is plenty but if you like it extra creamy use the fourth) Let thicken a bit.

Add 2/3 sauce to beef mixture along with egg yolk 2/3 of the mozzarella, set the rest aside.

A reminder to let the pasta and filling cool before trying to stuff it. It will heat back up in the oven.

Stuff manicotti with beef mixture and lay out in baking pan. Pour remaining sauce over them. (You may need to add a bit of milk over medium heat to thin it out a bit if it has cooled) Sprinkle with remaining mozzarella and parmesan cheese.

Bake at 400° for 25-30 minutes, or until cheese is golden. Let set for 5 minutes and serve.