In a large bowl, combine tomatoes, onions, cucumbers, and basil, season lightly with salt and pepper. Add olive oil and vinegar, and toss well to combine. Let stand at room temperature for at least 1 hour and up to 3 hours.
Heat broiler or grill pan. Cut bread diagonally into 1/2-inch-thick slices. Brush both sides lightly with olive oil; grill or broil on both sides until toasted.
Rub hot bread lightly with garlic cloves on 1 side only. Sprinkle with salt and pepper and top with tomato mixture. Let stand a few minutes to allow juices to soak into bread before serving.
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