In a large skillet, heat olive oil over med/med high heat. When hot, add beef and brown, crumbling as it cooks, for about 5-7 minutes, or until cooked through. Drain.
In the same skillet, add butter and heat over med heat. Add onion and bell pepper and cook about 3-4 minutes, until soft. Add garlic and cook about 30 seconds, until fragrant.
Add beef back to the skillet and add tomato paste. Stir well and cook for a minute or so, until the tomato paste is well incorporated.
Add ketchup, beef broth, brown sugar, mustard, chili powder, Worcestershire sauce, salt, red pepper flakes and pepper. Stir well to combine.
Cook over medium heat for about 7-9 minutes, until mixture has thickened a bit.