In a large skillet over medium-high heat, melt the butter or heat the olive oil.
Add the finely chopped onion and sauté for about 2-3 minutes until soft and translucent.
Add the green bell pepper and cook for another 2 minutes until softened and brightened in color.
Stir in the minced garlic and cook for about 30 seconds until fragrant.
Add the ground beef and break it apart as it cooks. Cook for about 5 minutes until no longer pink.
Stir in the ketchup, Worcestershire sauce, water, brown sugar, mustard, chili powder, salt, and black pepper.
Mix everything well and reduce the heat to medium. Let simmer for about 10 minutes, stirring occasionally.
Once thickened to your liking, remove from heat and serve on toasted hamburger buns.