Combine first 6 ingredients. Divide mixture into 10 equal portions, shaping each into a 1/2-inch-thick patty.
Place patties on a grill rack coated with cooking spray; grill 5 minutes on each side or until done. Place buns, cut sides down, on grill rack; grill 1 minute or until toasted. Spread 1 1/2 teaspoons ketchup on each bun top. Place 1 patty on bottom half of each bun; top each serving with 1 onion slice and top half of bun.