Preheat grill to medium-high (about 450°F). Combine first 5 ingredients in a small bowl.
Gently shape ground sirloin into 4 (4-inch-wide) patties. Sprinkle evenly with salt and pepper. Coat grill grate with cooking spray. Place patties on grate; grill 4 minutes on each side or to desired degree of doneness.
Place 1 lettuce leaf on bottom half of each bun. Top each with about 2 1/2 tablespoons sauce, 1 patty, 1 tomato slice, and 1 onion ring. Cover with top halves of buns.