Spray a 9x12-inch glass baking dish with cooking spray.
Use an additional baking dish if needed.
Drizzle olive oil or butter in the bottom of the baking dish.
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, SOFT bread crumbs (fresh bread frozen and then shredded), parsley, red and black pepper, and garlic salt.
Mix well and then stir in egg and mayonnaise.
Use a small cookie scoop to mound filling onto each mushroom cap.
Melt butter and mix in garlic salt and drizzle over the filled mushrooms.
Mound a generous portion of the shredded parmesan cheese on top of each filled mushroom. Note: For softer, melted cheese, you may want to use mozzarella cheese or brick cheese for the topping.
Bake for 20 to 25 minutes at 425°F.
Best served hot.
Can be prepared ahead, refrigerated and then baked just before serving (add 7-10 minutes additional baking time).