Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.
Clean the portobello mushrooms with a damp cloth and remove the stems. Place them on the baking sheet, gill-side up.
In a mixing bowl, combine the halved cherry tomatoes, diced mozzarella cheese, chopped basil, balsamic glaze, olive oil, garlic powder, salt, pepper, and red pepper flakes if using. Mix well until the ingredients are evenly coated.
Spoon the Caprese mixture generously into each portobello cap, pressing down gently to pack it in.
Bake in the preheated oven for 20-25 minutes, or until the mushrooms are tender, and the cheese is bubbly and slightly golden.
Remove from the oven and let cool for a few minutes before serving.
Drizzle with extra balsamic glaze if desired and garnish with additional basil leaves for a fresh touch.
Notes
Drizzle with extra balsamic glaze for added flavor.