Begin by preheating your oven to 350°F. Prepare a baking sheet by lining it with parchment paper. Spread the bread cubes on the sheet, drizzle with olive oil, and sprinkle with a pinch of sea salt. Toss the bread cubes to ensure they are evenly coated, then arrange them in a single layer. Bake for 7 to 10 minutes, or until the edges turn crisp and golden.
In a large mixing bowl, combine the olive oil, vinegar, grated garlic, Dijon mustard, salt, and a few generous grinds of black pepper. Whisk these ingredients together until the dressing is well emulsified.
Next, add the sliced red onion, tomatoes, cucumber, and the toasted bread cubes to the bowl with the dressing. Gently toss everything together to ensure the ingredients are well coated in the dressing. If desired, incorporate the torn mozzarella and fresh basil leaves, and give the salad a final gentle toss. Adjust the seasoning if needed. Serve the salad garnished with additional fresh basil for a vibrant finish.