Place garlic on a cutting board; hit it with the side of a large knife to break it open a bit. Place tomatoes, onion, and garlic in a large nonreactive bowl. Drizzle oil and vinegar over the vegetable mixture; season with salt and pepper. Toss; let stand, covered, in a cool place, for 1 hour or more.
Place bread on a hot grill or under a heated broiler; toast until both sides are slightly charred, 2 to 3 minutes. Remove from heat, rub lightly with garlic if desired; tear bread into bite-size chunks. When ready to serve, add cucumbers, basil, and bread to tomatoes.
Toss to coat bread thoroughly with the marinating liquid. Adjust the seasoning with salt and pepper, and remove garlic clove.
Variations: Try adding bocconcini (cubed fresh mozzarella), roasted bell peppers, sliced fresh fennel, or capers. Experiment using other seasonal vegetables.