Meanwhile, stir tomatoes, mozzarella, basil and the remaining 1/4 tsp salt and pepper and 1 Tbsp olive oil together in a medium bowl. Once the mushrooms have softened, remove from the oven and fill with the tomato mixture. bake until the cheese is fully melted and the tomatoes have wilted, about 12 to 15 minutes more. Drizzle each mushroom with 1/2 tsp balsamic glaze and serve.