Preheat oven to 425°F. Spray a 9x13-inch glass baking dish with cooking spray.
Wipe mushrooms with a damp paper towel to clean. Remove stems, set aside caps and chop stems to add to filling.
In a medium bowl, mix crabmeat, chopped mushroom stems, onion, dry mustard, 1 cup parmesan cheese, bread crumb, parsley, cayenne, black pepper, and 1/4 teaspoon garlic salt. Mix well and then stir in egg and mayonnaise.
Use a small cookie scoop to mound filling onto each mushroom cap. Slightly flatten out the top of the crab mixture to form a crab cake on the top of the mushroom cap.
In a small mixing bowl, stir remaining 1/4 teaspoon garlic salt into melted butter and drizzle over the filled mushrooms. Mound a generous portion of the shredded parmesan cheese on top of each filled mushroom.
Bake in a preheated oven for 20 minutes. Serve hot garnished with chopped chives and a squeeze of lemon.
Cook’s Note: Can be prepared ahead, refrigerated and then baked just before serving (add 7-10 minutes additional baking time).