Preheat your oven to 180°C (350°F). Place the large mushroom caps on a parchment-lined baking sheet. Bake for 10 minutes to start drying them out and make space for the filling.

While they bake, finely chop the mushroom stems and mince the garlic. Heat olive oil in a sauté pan over medium heat, then add the chopped stems and garlic. Cook for 3-4 minutes until fragrant and slightly browned, stirring often.

Transfer the cooked stems and garlic to a mixing bowl. Add the grated Parmesan, shredded mozzarella, bread crumbs, chopped parsley, and lemon juice (if using). Mix until well combined; the filling should be moist but not runny.

Remove the mushroom caps from the oven and let them cool slightly. Use a spoon to scoop out a bit of the inside if they are very large, creating a cavity for the filling, but leave enough to hold the mixture. Fill each cap generously with the prepared mixture, pressing lightly to pack the filling in.

Place the stuffed mushrooms back in the oven and bake for another 15-20 minutes until the tops are golden brown and bubbling. The filling should be hot and gooey, and the mushroom edges slightly shriveled and browned.

Remove from the oven and let them rest for 5 minutes to allow the filling to set. Garnish with additional parsley if desired, then serve hot, enjoying the rich aroma and crispy tops.