Combine the beef, 2 tablespoons of the barbecue sauce, salt and pepper. Form into 4 patties about the same diameter as the burger buns. Refrigerate until ready to grill.
Combine the mustard with the remaining barbecue sauce. Skewer the onion slices to prevent separating during cooking. Barbecue on medium heat brushing with a little of the sauce mixture. Barbecue the patties for about 4-5 minutes each side brushing with a little of the sauce. Barbecue the bacon, until crisp. Layer the ingredients on the bun bases and cover with the top.