Heat olive oil in a large skillet and toss in bread cubes. Toss frequently until lightly toasted (about 6 minutes). Remove from skillet and add to a large mixing bowl.
Add walnuts to skillet and stir as they toast for 1-2 minutes. Remove from heat and add to mixing bowl.
Add in thinly sliced red onion, sliced (or whole, if you prefer) tomatoes. I used a mixture of grape tomatoes and mini heirloom tomatoes. Add in sliced cucumber.
Brush peach halves with olive oil and grill, cut side down for 3-5 minutes (or until grill marks appear). Remove from grill and slice. Add to mixing bowl.
Whisk together olive oil and balsamic vinegar. Add to mixing bowl and toss to coat evenly. Spread panzanella onto platter or plates. Dot with mozzarella pieces and basil leaves. Top with salt and pepper to taste.