Heat a large skillet over medium heat. Cook and stir lean ground beef in the hot skillet until some of the fat starts to render, 3 to 4 minutes. Add onion and bell pepper; continue to cook until vegetables have softened and beef is cooked through, 3 to 5 more minutes.

Stir in ketchup, brown sugar, mustard, and garlic powder; season with salt and pepper. Reduce heat to low and simmer for 20 to 30 minutes.

Divide meat mixture evenly among hamburger buns.