Place a rack in the center of the oven. Preheat the oven to 350°F. With a paper towel, wipe off the outsides of mushrooms. Remove the stems and discard or use for soup or a stir fry (you can also freeze them). Set the caps aside.
In a medium mixing bowl, whisk together the Greek yogurt, egg yolk, garlic, and OLD BAY and salt.
Stir in the breadcrumbs, green onion, cheddar cheese, and parmesan cheese.
Add the crab meat.
With a fork or spatula, stir gently to incorporate it evenly.
With a spoon or small scoop, generously mound the filling into each mushroom cap. They will be very full. Arrange the stuffed mushrooms in a baking dish large enough to hold them snugly in a single layer (they will touch at the edges).
Bake the crab-stuffed mushrooms until browned and the filling is cooked through, about 20 to 25 minutes. Some liquid may seep out of the mushrooms; simply leave it behind in the pan.
Transfer the crab stuffed mushrooms to a serving plate and let cool at least 5 minutes. Top lightly with ground black pepper and an additional pinch of salt to taste. Sprinkle with fresh parsley. Enjoy warm or at room temperature.
Notes
TO STORE: Refrigerate stuffed mushrooms in an airtight storage container for up to 2 days.
TO REHEAT: Gently rewarm leftovers in a baking dish in the oven at 350°F or in the microwave.
TO FREEZE: I don't recommend freezing cooked stuffed mushrooms, as they tend to be soggy once reheated. If you have leftovers and want to give it a try, freeze them in an airtight freezer-safe storage container for up to 3 months. Reheat from frozen.
TO MAKE AHEAD: Prepare the mushroom filling and stuff the uncooked mushrooms as directed. Lay the uncooked stuffed mushrooms on a baking sheet and freeze until solid. Transfer the frozen crab stuffed mushrooms to an airtight freezer-safe storage container for up to 3 months. Bake from frozen as directed, adding a few extra minutes to the bake time if needed.