Combine the ground beef, onion, breadcrumbs, egg, and 1/3 cup barbecue sauce in a large bowl. Shape the mixture into a loaf and wrap in cooking-spray-coated aluminum foil. Chill for 15 minutes in the freezer.
Preheat the grill.
Poke holes on both sides of the foil using a fork. Grill the meatloaf, still in the foil package, covered, over medium-high heat for 10-15 minutes per side. It should be taken off the grill once its internal temperature reaches 155 degrees F.
Open the foil and spoon 1/4 cup barbecue sauce over the meatloaf. Cover with aluminum foil for 5-10 minutes to heat through. Serve immediately.