1. Prepare grill to medium-high heat.
2. Combine first 6 ingredients, stirring with a whisk. Cover and chill.
3. Coat lettuce on all sides with cooking spray; sprinkle with salt and pepper. Place lettuce, cut side down, on grill rack; grill 2 minutes. Remove lettuce from grill; let stand 5 minutes.
4. Place 1 lettuce half on each of 4 salad plates. Top each serving with 1/4 cup tomatoes. Drizzle each serving with 1 tablespoon dressing. Serve immediately.