How to Make It
Place a rimmed baking sheet on top rack of oven; preheat oven to 450°F. (Do not remove baking sheet while oven preheats.) Toss onion wedges and garlic with 1 tablespoon of the olive oil and 1⁄4 teaspoon each of the salt and pepper. Spread onion mixture in a single layer on preheated baking sheet; place on top oven rack. Place bread cubes on a separate large rimmed baking sheet; place baking sheet on center rack in oven. Bake in preheated oven until bread is toasted and onions are softened and charred, 15 to 17 minutes, stirring onions after 8 minutes.
Meanwhile, use a spoon to scrape seeds and juices from heirloom tomato wedges into a medium bowl. Place scraped wedges in a large bowl, and set aside.
Whisk vinegar, mustard, and remaining 1⁄2 teaspoon salt and 1⁄2 teaspoon pepper into tomato juices and seeds. Slowly whisk in remaining 7 tablespoons olive oil until emulsified.
Place toasted bread, onion mixture, chicken, cucumber, cherry tomatoes, and peppers in bowl with tomato wedges. Drizzle vinaigrette over salad; toss gently to coat. Let stand 15 minutes. Top with herbs.