The flavor of charred bread is the key to this bright, bubbly gazpacho, so use the best you can find. I like using sweet cherry tomatoes here because you can chuck them into the blender with no prep, but if you have super ripe beefsteak or heirloom tomatoes, give them a rough chop and use those instead.
Prepare your charcoal or gas grill for direct, high-heat grilling. (Or heat your broiler to high and line a baking sheet with foil.) Brush the bread all over with olive oil, season with salt and pepper, and grill or broil the bread, flipping once halfway through, until deep golden brown all over with some charred bits at the edges, two to three minutes. Remove the bread from the grill or broiler and let cool. Tear into bite-size chunks.
Place the grilled bread in a blender along with one-half cup olive oil, cherry tomatoes, chopped cucumbers, roughly chopped red onion, sherry vinegar and garlic. Blend until smooth, then pour into a large bowl and refrigerate until chilled, at least one hour.
Cut the remaining six cherry tomatoes into three slices each. Place the remaining minced red onion in a sieve, rinse under cold water for about 10 seconds, then drain.
Remove the soup from the fridge, stir in the seltzer, then season with more salt and sherry vinegar to taste. Pour the soup into serving bowls and garnish each with three slices of cherry tomato in the center and then scatter the rinsed red onions and finely diced cucumber over the top. Drizzle with more olive oil and sprinkle with a pinch of flaky sea salt to serve.
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