Slice the tomatoes into ½-inch thick slices. Arrange them in a single layer on a baking sheet.
Season the tomatoes with the sea salt and black pepper, then drizzle with 2 tablespoons of balsamic vinegar.
Bake for 10–15 minutes, or until the tomatoes are softened.
Meanwhile, prepare the balsamic reduction. In a small saucepan, combine the ½ cup balsamic vinegar and brown sugar. Bring to a gentle simmer over medium heat, whisking until the sugar dissolves.
Continue simmering for 8–10 minutes, or until the mixture has reduced by about half and is thick enough to coat the back of a spoon. Remove from the heat and let it cool slightly.
Slice the fresh mozzarella into ½-inch thick slices and thinly slice the basil leaves.
Arrange the roasted tomatoes on a serving platter, alternating with slices of fresh mozzarella.
Top with the sliced basil and drizzle with the balsamic reduction.