Soaking the onion takes away its bite and pulls its sweetness to the forefront. Recipe adapted from A Bird in the Oven by Mindy Fox (Kyle Books; 2010).
Preheat oven to 450 degrees. In a small bowl, cover onion with cold water. Swish water around, rubbing onion slices with your hand. Drain and repeat 2 or 3 times, letting slices soak and changing water every 10 minutes. Drain onion; pat dry. Arrange bread in a single layer on a rimmed baking sheet. Bake until edges are crisp and golden, 6 minutes. Let cool completely on sheet on a wire rack; transfer to a large bowl. Add tomatoes and 1/2 teaspoon salt; let sit 10 minutes.
Meanwhile, finely chop garlic with a generous pinch of salt. Drag the blade of a large knife at an angle across mixture until a paste forms. Transfer to a small bowl and whisk in oil and vinegar. To tomato mixture, add onion, cucumber, celery, radishes, basil, parsley, capers, and dressing; toss to combine. To serve, season with salt and pepper and toss.