Preheat a skillet with oil and add sliced onions. Cook until the onions turn golden brown and set aside.
Using the same skillet, cook beef until it turns golden brown and set aside.
Using a large bowl, combine beef, onion and cheese. Season with salt and pepper and toss it all together.
Slice the bell peppers into halves and remove the seeds from inside. Stuff each pepper with a steak filling. Bake the bell peppers at 400 °F for 25 minutes.
Sprinkle the remaining cheese on top of each pepper and bake for another 5 minutes.
Serve the dish right away while the cheese is melted.