Mix the beef, onion, breadcrumbs, egg, and 1/3 cup barbecue sauce in a large bowl. Shape the mixture into a loaf. Wrap it in cooking-spray-coated aluminum foil and chill for 15 minutes in the freezer.
Preheat the grill.
Poke holes on both sides of the foil using a fork. Grill the meatloaf in the foil package, covered, over medium-high heat for 10-15 minutes per side. The internal temperature should be 155 degrees F. when the meatloaf is removed.
Open the foil and spoon 1/4 cup barbecue sauce over the meatloaf. Cover again with aluminum foil 5-10 minutes in order to heat through. Serve immediately.