Step 1

Combine halved tomatoes, finely chopped onion, 3 tablespoons olive oil, chopped fresh basil, and balsamic vinegar in medium bowl; season tomato topping with salt and pepper. DO AHEAD Topping can be made 2 hours ahead. Cover and let stand at room temperature, tossing occasionally.

Step 2

Preheat broiler. Place focaccia half, cut side up, on baking sheet; brush bread with olive oil. Toast bread in broiler until top is golden brown, watching closely to avoid burning, about 2 minutes. Immediately rub cut side of bread all over with halved garlic, then cut focaccia crosswise into 4 equal pieces.

Step 3

Top toasted focaccia with tomato mixture and serve.