Rinse and core the tomatoes. Cut into wedges or thick rounds, mixing sizes and colors for variety.
Place the tomato pieces in a 30 cm / 12-inch shallow bowl. Sprinkle with half the flaky salt and let sit for 10 minutes at room temperature. This draws out the sweet juices that will become the dressing.
While the tomatoes rest, tear or halve the mozzarella and set it aside.
In a small bowl, whisk together the olive oil, balsamic vinegar, grated garlic, and black pepper.
If using red onion, drain it from the cold water and pat dry.
Tuck the mozzarella pieces among the tomato wedges in the bowl, pressing them gently into the pooled juices.
Scatter the drained onion and torn basil leaves over the top.
Drizzle the dressing evenly over the bowl. Finish with the remaining flaky salt and an extra crack of black pepper.
Serve right away with crusty bread on the side for mopping up the juices.