Arrange bread slices in a single layer on a baking sheet. Bake at 425° for 8 minutes, turning after 4 minutes. Rub one side of toast with cut side of 1 garlic clove half; set toast aside. Discard garlic clove half. Mince remaining garlic clove half.
Heat oil in a medium nonstick skillet over medium heat. Add minced garlic to pan; cook 30 seconds, stirring constantly. Add tomato and onion to pan; cook 30 seconds, stirring constantly. Remove pan from heat. Stir in vinegar and capers. Cool completely. Stir in basil, salt, and pepper. Spoon 1/3 cup tomato mixture onto each toast slice. Serve immediately.