Preheat oven to 350F. In a small greased bread (or meatloaf) pan, line the bottom and sides with about 2/3 of the ground beef, leaving the center open for your stuffing.
Layer the cheese on the bottom of the meatloaf.
Add in the onions, spinach and mushrooms.
Use the remaining meat and spread evenly on top, covering the spinach and mushrooms so that it acts as a lid.
Bake for one hour, then remove from oven and let rest for 5 minutes in pan. Remove to serving platter, then slice and serve with your favorite side!