Preheat the oven to 375 degrees F. Lightly grease a 9x5-inch loaf pan.
Combine ground beef, onion, milk, bread crumbs, bbq sauce and egg in a large bowl; season with salt and pepper. Transfer into prepared loaf pan.
Mix remaining BBQ sauce and mustard together in a small bowl until well combined; pour over meatloaf and spread it evenly over the top.
Bake in the preheated oven until no longer pink in the center, about 45 minutes or until 175°
