In a skillet, melt the butter over medium heat. Add the onions, mushrooms and garlic, and saute until they are golden and all liquid is absorbed (about 20-25 minutes), stirring occasionally. Cut roast beef into thin strips and add to the mushroom mixture. Add salt and pepper. Stir well and cook for 1-2 minutes. Set the mixture aside.
Preheat oven to 400 degrees F. Slice bell peppers in half lengthwise and remove seeds. Line the inside of each bell pepper with half a slice of mozzarella cheese.
Fill each pepper with mushroom-beef mixture until they are nearly overflowing.
Then top each one with a slice of mozzarella cheese and place them in a casserole dish.
Bake in the oven until cheese is melted and browned, about 15-20 minutes.