Add the ground beef to a large skillet (this browns very well) on high heat and brown until a deep brown crust appears before breaking the beef apart.
Stir the ground beef and brown until a deep crust appears on about 50 or so percent of the beef, and it is cooked through, then remove the beef, drain the excess liquid and add the butter, onions and bell pepper to the pan.
Let brown for 1-2 minutes before stirring, then let brown for another 1-2 minutes before stirring again and add the beef back into the pan.
Add the garlic, mustard, ketchup, brown sugar, salt, black pepper, to the pan and cook until most of the liquid has evaporated (about 75% of the mixture is above liquid), 3-5 minutes.
Serve on toasted hamburger buns.
