Prepare the ingredients for the recipe.
Preheat skillet with oil. Add mushrooms and cook until they’re slightly browned. Add diced onions and cook until golden brown. Remove form the skillet and set aside.
In a same skilled, brown sliced beef. Add the mushroom mixture and season with salt and pepper to taste.
Slice bell peppers in half, clean out the inside. Place one slice of cheese on the inside of peppers, top it off with beef mixture and cover with cheese.
Bake stuffed peppers at 400℉ for 25 minutes.