Chop and sautee onions in a little olive oil or butter
In a large mixing bowl -mix ground beef, bread crumbs, eggs, and mushrooms
add onions and mix again
With ⅔-3/4 of the meat mixture, line a glass loaf pan, forming a cavity in the middle
add half of the cheese to the cavity
Top with the remaining meat mixture to cover the cavity and the cheese
Add the remaining cheese on top
Cover with foil and bake at 400° for 45 minutes
Take foil off, add tomato sauce to the top of the meatloaf
Bake another 15 minutes
Drain excess grease from meatloaf and let cool for 10-15 minutes before cutting