Prep the mushrooms: Heat the oven to 400°F (200°C). Line a baking sheet with parchment. Wipe mushrooms clean with a damp towel. Gently twist out the stems, then finely chop the stems and set aside. Lightly brush the mushroom caps with olive oil and season with a pinch of salt and pepper.

Pre-roast the caps: Arrange caps cavity-side down on the baking sheet. Roast for 8–10 minutes to release excess moisture. Blot any pooled liquid with a paper towel. Flip the caps cavity-side up and let cool slightly.

Sauté the stems and garlic: In a skillet, warm 2 tbsp olive oil over medium heat. Add chopped mushroom stems and a pinch of salt. Cook 3–4 minutes until they release moisture and turn golden. Stir in garlic and cook 30 seconds until fragrant. Remove from heat and let cool 2 minutes.

Mix the filling: In a bowl, combine sautéed stems, tomatoes, mozzarella, basil, red pepper flakes (if using), Parmesan (if using), and breadcrumbs (if using). Season with salt and pepper. Add 1–2 tsp balsamic and toss gently to coat without crushing the tomatoes.

Stuff the mushrooms: Spoon the filling into each cap, packing it slightly so the cheese and tomatoes sit snugly. Place the stuffed caps back on the baking sheet.

Bake: Roast for 10–12 minutes, until the mozzarella melts and edges start to brown. For a golden top, switch to broil for 1–2 minutes, watching closely.

Finish and serve: Transfer to a platter. Drizzle lightly with balsamic glaze, add a few basil ribbons, and a grind of black pepper. Serve warm.