In a skillet, melt butter over medium heat. Add the onions, mushrooms and garlic, and cook until all liquid is absorbed, about 20-25 minutes.

Cut the roast beef into thin strips and add to the mushroom mixture. Add salt and pepper to taste. Stir and cook for 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.