Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper.
In a mixing bowl, combine the halved cherry tomatoes, mozzarella balls, and chopped basil. Season the mixture with salt, pepper, garlic powder, and red pepper flakes (if using). Drizzle with balsamic glaze and toss gently to combine.
Arrange the portobello mushrooms on the prepared baking sheet, gill side up. Brush the tops with olive oil and season with a pinch of salt and pepper.
Spoon the Caprese mixture evenly into each portobello cap, pressing down slightly to pack it in.
Bake in the preheated oven for about 20-25 minutes, or until the mushrooms are tender and the mozzarella is melted and slightly golden.
Remove from the oven and drizzle with additional balsamic glaze before serving.
Notes
Serve the stuffed portobellos on a rustic wooden platter, garnished with extra fresh basil leaves and a drizzle of balsamic glaze for a beautiful finish.