Warm butter in a skillet over medium heat. Sauté onion for about 1 minute until fragrant, then add the bell pepper and continue cooking for another 4 minutes until soft.
Raise heat to medium-high and add ground beef, seasoned with salt and pepper. Break apart the meat with a spatula, if needed. Brown the meat for 5-7 minutes until no longer pink.
Top the meat and vegetable mixture with shredded cheese.
Remove from heat and cover with a lid for about 5 minutes until the cheese is melted. Serve immediately.
