Combine the onion, 2 tablespoons barbecue sauce, breadcrumbs, and the next 5 ingredients (breadcrumbs through turkey) in a medium bowl. Divide turkey mixture into 4 equal portions, shaping each into a 1 1/2 inch-thick patty.
Heat a grill pan coated with cooking spray over medium-high heat. Place patties in pan; cook 7 minutes on each side or until done.
Place 1 lettuce leaf, 1 tomato slice, and 1 patty on the bottom half of each bun. Spread each patty with 1 1/2 teaspoons barbecue sauce. Cover with top halves of buns.