Preheat your oven to 375 degrees Fahrenheit. Line a large baking sheet with parchment paper or lightly grease it with a bit of oil to prevent the mushrooms from sticking during the roasting process.
Carefully clean the mushrooms using a damp paper towel to remove any dirt. Gently pop the stems out of the mushroom caps. You can discard the stems or finely chop them to include in the filling if you prefer a bit more texture. Place the hollowed caps on the prepared baking sheet with the open side facing up.
In a medium mixing bowl, combine the softened cream cheese, shredded crab meat, green onions, minced garlic, parmesan cheese, Old Bay seasoning, salt, and pepper. Use a spatula or a large spoon to fold everything together until the mixture is smooth and the crab is evenly distributed throughout the cheese.
Using a small spoon or a piping bag, generously fill each mushroom cap with the crab and cream cheese mixture. In a separate small bowl, toss the panko breadcrumbs with the melted butter until they are evenly coated. Sprinkle a small amount of the buttery breadcrumbs over the top of each stuffed mushroom, pressing lightly so they stick.
Place the baking sheet in the center of the oven and bake for 20 minutes. The mushrooms should be tender and the tops should be golden brown and slightly bubbly. Remove from the oven and let them rest for 5 minutes before garnishing with fresh parsley and serving.
