Combine first 5 ingredients; set aside. Combine mayonnaise and next 3 ingredients; set aside.
Place fish on a grill rack coated with cooking spray; grill, covered, 5 minutes on each side or until fish flakes easily when tested with a fork, basting with lemon juice mixture.
Layer 1 lettuce leaf, 3 slices tomato, and 1 piece fish on bottom half of each bun. Spoon mayonnaise mixture evenly onto fish, and cover with top halves of buns. Serve immediately.