Saute onions and garlic in the olive oil until the onions are translucent. Add the steak and a little salt and pepper and cook until nearly done. When there is a little pink left in the meat, add the mushrooms and cook for about 5 more minutes.
Meanwhile, remove the tops, ribs, and seeds from the green peppers, and blanch the green peppers in a pot of boiling water for about 5 minutes. Remove with tongs and place upside down on a drying rack with a towel underneath.
Spray the inside of a 8 X 8 baking pan or a large bread pan with nonstick cooking spray. Place the peppers in the pan (openings facing up). Tear 4 of the pieces of cheese into strip and place inside each pepper. Then fill them with the steak and onion mixture and place another slice of cheese on top.
Cover the pan with tented aluminum foil and bake at 350F for 30 minutes. Then remove foil and bake about another 10 minutes, until cheese is slightly browned and bubbly. Enjoy!