Look for packaged hearts of romaine with the bags of prewashed salad greens in your supermarket's produce section. If available, use multicolored teardrop tomatoes for a beautiful peak-of-summer presentation.
Prepare grill to medium-high heat.
Combine first 6 ingredients, stirring with a whisk. Cover and chill.
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.
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.
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