In a bowl, combine the diced tomatoes, minced garlic, red onion, extra-virgin olive oil, red wine vinegar, salt, basil, and pepper. Once this mixture is complete, cover the bowl with a cloth, foil, or plastic wrap. Set to the side.
Rub bread slices with garlic halves. Lightly brush the top (or both sides if desired) with olive oil. Toast or grill bread on both sides until bread is slightly crispy. Once the bread is toasted, top it off with a slice of fresh mozzarella cheese. Add tomato mixture as desired.