1. Pre-heat your oven to 350 degrees. Line a large baking sheet with parchment with parchment.
2. In a large bowl, toss the bread cubes with the olive oil and generous pinch of salt and pepper. Transfer to the prepared baking sheet. Bake until crisp, firm and very lightly browned, about 15 minutes. Remove from the oven and let cool completely.
3. Meanwhile, whisk the red wine vinegar, garlic, mustard, and crushed red pepper (if using) in a medium bowl. Slowly drizzle in the olive oil, whisking constantly, until fully emulsified (aka well incorporated and smooth). Season generously with salt and pepper.
4. In a large bowl toss the toasted bread cubes, tomatoes, nectarines, cucumber, red onion, and basil. Drizzle with the dressing and gently toss to coat. Add the mozzarella, and gently toss again.
5. Let the salad rest for about 20 minutes for the flavors to blend before serving. (feel free to drizzle the salad with a little extra olive oil and vinegar if you like.)