Step 1

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.  Transfer to colander to drain.

Step 2

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.

Step 3

Add ketchup, water, brown sugar, mustard, chili powder, Worcestershire sauce, salt, red pepper flakes (if using), and black pepper.  Stir well to combine.

Step 4

Cook over MED heat for 10-15 minutes, until mixture has thickened to your liking.  Remove from heat and serve over toasted buns.