Preheat the oven to 400°F (200°C) and lightly grease a baking dish.
Finely chop the reserved mushroom stems and sauté with minced garlic in 1 tablespoon of butter until softened, then let cool.
In a medium mixing bowl, combine the cream cheese, sautéed stems, Parmesan cheese, lemon juice, Old Bay seasoning, salt, and pepper until smooth.
Gently fold in the lump crab meat and green onions, maintaining the integrity of the crab lumps.
Fill each mushroom cap generously with the crab mixture and place them in the prepared baking dish.
In a small bowl, mix the panko breadcrumbs with the remaining melted butter and sprinkle over the tops of the stuffed mushrooms.
Bake for 15 minutes or until the mushrooms are tender and the breadcrumb topping is golden brown.
Garnish with fresh parsley and serve immediately.