Add the balsamic vinegar to a small saucepan over medium heat. Bring to a simmer and cook until syrupy and thick, about 15 minutes. Set aside to cool.
Slice the tomatoes and mozzarella into 1/2-inch-thick slices, and alternate the slices on a platter. Season with salt and pepper. Drizzle with the balsamic reduction and some olive oil. Garnish with torn basil leaves.