Prepare the mushrooms. Preheat your oven to 400°F (200°C) and prepare a baking sheet. Carefully remove the stems from the mushrooms, then finely chop them and set aside.
Cook the mushrooms. Heat the butter in a skillet over medium heat. Add the finely chopped mushroom stems and sauté for about 5 minutes, stirring occasionally. Stir in the onion, garlic, salt, and pepper, and continue cooking for 1–2 minutes, or until the onion becomes soft and fragrant. Transfer the mixture to a bowl and set aside.
Make the filling. In a bowl, combine the cream cheese, Parmesan (reserve 1 tablespoon), parsley, and breadcrumbs (reserve 1 tablespoon). Stir until well combined.
Fill the mushrooms. Fill each mushroom cap generously with the cheese mixture. Sprinkle the reserved Parmesan and breadcrumbs evenly over the stuffed mushrooms.
Bake the mushrooms. Bake for 15 to 20 minutes, or until the top is golden and the mushrooms are slightly soft. Garnish with extra grated Parmesan and serve warm.