Fill pot with water and bring to a boil.
Wash, stem and remove seeds from peppers. Place them in the boiling water and push them down to the bottom of the pot. Boil for 15-20 minutes (uncovered) until peppers are soft.
While peppers are boiling brown meat
Add onion, salt and pepper, and Italian Seasoning.
Once meat is browned and no longer pink, add tomato sauce, tomatoes, rice, and mushrooms to the meat mixture and stir gently.
Simmer on low for 15-30 minutes.
Line dish with aluminum foil, and spray with cooking spray.
Add peppers to dish and begin scooping meat mixture inside each pepper until it's filled to the top. Leave enough room to add the cheese later.
Bake at 350 uncovered for 20 minutes or until you can see the edges turn a little brown.
Remove peppers and add cheese to the top of each one. Place back in the oven until cheese is melted completely, about 15 minutes depending on your oven.