Heat olive oil in a large skillet over medium high heat. Add ground beef and brown, breaking it up with spatula into crumbles as it cooks, about 5 minutes, or until no longer pink. Transfer to a colander to drain.

Add chopped onion and bell pepper to the same skillet and cook 2-3 minutes, until soft. Add garlic and cook for 30 seconds, until fragrant.

Add tomato paste, ketchup, water, brown sugar, mustard, chili powder, Worcestershire sauce, salt, red pepper flakes (if using), and black pepper. Add beef back to the skillet and stir well to combine.

Cook over medium heat for about 5 - 10 minutes, until mixture has thickened to your liking. Remove from heat and serve over buns.