In a large mixing bowl, combine the halved cherry tomatoes, mozzarella balls, torn basil leaves, minced garlic, balsamic vinegar, olive oil, and red pepper flakes (if using). Season with salt and pepper, then gently toss until well mixed.
Place the portobello mushrooms, gill side up, on a baking sheet lined with parchment paper.
Spoon the Caprese mixture into each portobello cap until generously filled.
Sprinkle grated Parmesan cheese over the stuffed mushrooms for an added layer of flavor.
Bake in the preheated oven for 20-25 minutes, or until the mushrooms are tender and the cheese is bubbly and slightly golden.
Once done, remove from the oven and let cool for a couple of minutes before serving.
Notes
Serve the stuffed portobellos on a bed of fresh arugula for a colorful contrast. Drizzle with extra balsamic glaze for a gourmet touch!