Prep the ingredients: Finely dice the onion and bell pepper, mince the garlic, and measure the sauce ingredients before cooking.
Brown the beef: Heat oil or butter in a large skillet over medium-high heat. Add ground beef and cook for 5–7 minutes, breaking it into crumbles. Drain excess grease if needed.
Cook the vegetables: Add onion and bell pepper to the skillet. Cook for 3–5 minutes until softened, then stir in garlic and cook for 30 seconds.
Add the sauce: Stir in tomato paste, tomato sauce, ketchup, Worcestershire sauce, mustard, brown sugar, chili powder, salt, and pepper until the beef is well coated.
Simmer: Reduce heat to medium-low and simmer uncovered for 10–15 minutes, stirring occasionally, until the filling is thick and saucy.
Toast the buns: Toast the hamburger buns cut-side down for 2–3 minutes, until lightly golden.
Assemble and serve: Spoon the hot filling onto toasted buns, add your favorite toppings, and serve warm.
