Make the salad. Preheat the oven to 350°F and line a baking sheet with parchment paper.
Spread out the bread on the sheet and drizzle with the olive oil. Bake for 15 minutes or until golden brown and toasted, then let cool.
Meanwhile, make the dressing. In a medium bowl, whisk together the Dijon and vinegar, then gradually whisk in the olive oil. Season to taste with salt and pepper.
In a large bowl, add the bread, tomatoes, cucumbers, onion, basil, mozzarella, and capers. Add about half the dressing, then toss to combine. Season and adjust dressing to taste.
Transfer to a serving platter and top with the prosciutto. Serve any remaining dressing on the side.