01 - Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper for easy cleanup.
02 - Brush the portobello mushroom caps on both sides with olive oil. Season them generously with salt and freshly ground black pepper. Place them gill-side up on the prepared baking sheet.
03 - In a medium mixing bowl, combine the shredded mozzarella, ricotta cheese, grated Parmesan cheese, minced garlic, chopped parsley, chopped basil, dried oregano, and chili flakes (if using). Mix until all ingredients are well incorporated.
04 - Evenly spoon the prepared cheese mixture into each mushroom cap. Gently press down to ensure the filling is compact and fills each cap completely.
05 - In a separate small bowl, stir together the breadcrumbs and melted butter until the breadcrumbs are moistened. Sprinkle this breadcrumb mixture evenly over the stuffed mushrooms.
06 - Bake for 20 to 25 minutes, or until the cheese is gloriously bubbling and the tops of the mushrooms are golden brown and inviting.
07 - Allow the stuffed mushrooms to cool for 5 minutes before serving. This allows the flavors to meld and makes them easier to handle.