Heat butter and oil in large skillet over MED/MED-HIGH heat. Add beef and brown, breaking apart into crumbles as it cooks, about 5 minutes. Drain.
Add onion and bell pepper to same skillet and cook 2-3 minutes, until soft. Add garlic and cook 30 seconds or so, until fragrant. Add beef back to the skillet and add tomato paste. Stir well.
Add ketchup, water, brown sugar, mustard, chili powder, Worcestershire sauce, salt, red pepper flakes (if using), and black pepper. Stir well to combine.
Cook over MED heat for 10-15 minutes, until mixture has thickened to your liking. Remove from heat and serve over toasted buns
