Toss bread with 2 tablespoons olive oil and toast in a 400°F oven 8–10 minutes until golden. Watch it; I once burned half a loaf while folding laundry.

Whisk remaining oil, vinegar, mustard, salt, and pepper in a bowl.

Combine tomatoes, cucumber, onion, basil, and mozzarella in a large bowl.

Add toasted bread and dressing. Toss gently and let sit 10 minutes so bread soaks up juices.