Slice the bread into cubes. On a rimmed baking sheet, toss the cubes with 1 tablespoon olive oil and season with kosher salt and fresh ground black pepper. Spread in a single layer and bake until golden brown, about 7 minutes.
In a medium bowl, combine the sherry vinegar, and garlic. Gradually whisk in 3 tablespoons olive oil, 1 tablespoon at a time, until a creamy emulsion forms. Season with kosher salt and fresh ground black pepper.
In a large bowl, combine the chopped vegetables, dressing, basil, and toasted bread. (If making in advance, add the toasted bread last to avoid becoming soggy.) Allow to stand about 15 minutes for the flavors to meld, then serve immediately. (Leftovers are delicious, but the bread becomes soggy after a few hours.)