Make your croutons. Preheat your oven to 400 degrees. Cube your bread and arrange on a sheet tray with olive oil, salt and pepper. Toast for 15-20 minutes until golden brown. Toss halfway through. Remove and let cool.
Begin your ingredient prep! Cut your tomatoes in wedges, and add in a colander over another bowl. Sprinkle salt and let the tomatoes salt and soak for about 15-20 minutes. Reserve and don't throw out the liquid.
Slice your cucumbers, and thinly slice red onions: add your red onion in a bowl with water, this helps mellow out the onion-y raw flavor and gives more crunch.
Make your dressing. In the same bowl as the reserved salted tomato juice, add in dijon mustard, grated garlic, sherry vinegar, pepper and olive oil. Mix until emulsified.
Adjust with salt if needed. Since the tomato juice has salt in it, you may not need any additional salt for the dressing but adjust to your preference. Mix well and set aside.
Assemble. Lastly, add all your ingredients in a bowl: the tomatoes, cucumber, red onion (remove from water), croutons, basil, mozzarella and a little bit more freshly cracked pepper.
Finish by tossing in your dressing, mix and serve!