Preheat a deep skillet with oil. Add mushrooms and cook until they’re slightly browned. Add diced onions and cook until golden brown. Remove the veggies from the skillet and set aside.
In a same skillet, brown the sliced beef. Add the veggie mixture back into the pan and season with salt and pepper to taste.
Slice bell peppers near the top and clean out the inside. Line one slice of cheese on the inside of each pepper, top them off with the beef mixture, and cover with another slice of cheese.
