Place tomato wedges in a large bowl and sprinkle with salt, then let sit for 5 minutes to release juices.
In a small bowl, whisk together olive oil, balsamic vinegar, minced garlic, and black pepper until well combined.
Add mozzarella cubes, torn basil leaves, and sliced red onion to the tomatoes and gently toss to combine.
Pour the dressing over the salad and toss gently to coat all ingredients.
Let the salad sit at room temperature for 10-15 minutes before serving to allow flavors to meld together.