FOR THE GARLIC BUTTER SAUCE (optional)
Preheat oven to 375F.
Lightly grease a large baking dish with cooking spray and arrange the mushroom caps, stem-side-up. Set aside.
In a large bowl, combine crabmeat, green onions, panko crumbs, parmesan cheese, parsley, salt and pepper; mix to combine.
Add milk and continue to stir until mixture is just moistened. IF it's too dry, add a tablespoon of milk.
Spoon the filling into the prepared mushrooms.
Transfer to oven and bake for 15 minutes.
In the meantime, prepare the Garlic Butter Sauce:
Melt butter in a saucepan.
When completely melted, remove from heat and, using a spoon, remove the foamy layer on top; discard foam.
Whisk in garlic powder and salt.
Remove mushrooms from oven and let stand 1 minute.
Drizzle with prepared garlic butter sauce.