Preheat oven to 350 degrees
Spay 9x13 pan with cooking spray.
Slice the top off of each pepper and remove stem and membranes from insides. Rinse peppers.
In a large stock pot or sauce pan, place peppers in pan and add enough water to cover all of the peppers. Bring to boil, about 5 minutes.
With tongs, remove peppers from boiling water, drain, and put peppers in baking pan, open side up.
In medium-sized skillet, brown the ground beef and onion over medium heat until beef is brown (about 8-10 minutes). Drain excess grease.
Add cooked rice, salt, garlic, and 1 cup of the tomato sauce to the beef. Cook until warmed through.
Spoon meat and rice mixture into each pepper, filling to the top.
Pour remaining tomato sauce on top of peppers.
Cover and bake 45 minutes. Uncover and bake about 15 more minutes, or until peppers are tender.
Sprinkle with mozzarella cheese.