Preheat a large, high-sided pan over medium heat. Add oil to the pan. Once hot, add the sliced bell peppers and onion. Cook, stirring occasionally, for about 15 minutes or until tender and onions are becoming translucent. Remove cooked peppers and onions from the pan and set aside.
Add the ground beef to the pan and use a spatula to break it into small pieces. Continue cooking for about 5-7 minutes, or until the beef is almost cooked through. Drain excess grease from the pan.
Add back in the peppers and onion mixture, minced garlic, and season with salt and pepper. Stir to combine.
Lower heat to medium-low. Sprinkle the cheese evenly across the skillet. Cover with a lid and let cook for about 5-10 minutes or until the cheese is fully melted.
Serve as is or with bread! Leftovers can be stored in an airtight container in the fridge for a few days.