This variation on the classic bread salad panzanella depends on ripe seasonal tomatoes for its success.
Core tomatoes and cut them into 3/4-inch chunks. Cut cucumber halves cross-wise into 1/4-inch slices. Mince 1 clove garlic. Place tomatoes, cucumber, onion, and minced garlic in a mixing bowl and toss with oil, vinegar, and salt and pepper to taste. Let marinate at room temperature for at least 1 hour.
Trim bottom and top crusts from loaf. Cut loaf horizontally into 3 round slices. Place bread rounds on a hot grill or under the broiler and toast on each side until brown around edges. Remove from heat and rub one side of each toast with remaining garlic clove. Brush with oil and season with salt and pepper.
Cut basil leaves into thin strips and toss with tomato mixture. Spoon tomato salad, including juice, over grilled bread. Cut bread into wedges and serve immediately as an appetizer or with a meal.