Preheat your oven to 375°F (190°C).

Clean the mushrooms with a damp paper towel to remove any dirt. Gently twist and remove the stems, setting them aside for later use.

Arrange the mushroom caps, hollow side up, on a baking sheet lined with parchment paper or lightly greased.

Finely chop the mushroom stems.

In a large skillet, heat the butter and olive oil over medium heat.

Add the chopped onion and mushroom stems to the skillet. Sauté for about 5 minutes, or until softened.

Stir in the minced garlic and cook for an additional 1 minute, until fragrant. Remove from heat and let cool slightly.

In a mixing bowl, combine the sautéed mixture with the crabmeat, cream cheese, ¼ cup Parmesan cheese, breadcrumbs, parsley, Old Bay seasoning, and a pinch of salt and pepper. Mix gently until well combined.

Using a small spoon or your hands, fill each mushroom cap generously with the crab mixture, pressing lightly to ensure it stays in place.

(Optional) For a crispy topping, mix the melted butter, breadcrumbs, and Parmesan cheese in a small bowl. Sprinkle this mixture over the stuffed mushrooms.

Place the baking sheet in the preheated oven and bake for 20-25 minutes, or until the mushrooms are tender and the filling is golden brown.

Remove from the oven and let cool for 5 minutes. Garnish with additional chopped parsley if desired.

Transfer the stuffed mushrooms to a serving platter and serve warm. Perfect as an appetizer for parties, holidays, or cozy dinners!