Preheat oven to 350º F.
Line a baking tray with parchment paper.
Wash mushrooms in cold water and pat dry. Remove the stems, finely chop them and set aside.
Place uncooked sausage, onion and garlic in a large skillet and cook on medium heat until brown and onion and garlic are soft (break into pieces as the mixture is cooking).
Drain sausage mixture and let cool.
Add chopped mushroom stems to the same skillet and cook for 2 minutes over medium-low heat.
Combine mushroom and sausage mixture in a bowl.
Add ½ cup Parmesan cheese and seasoned panko crumbs.
Add the egg yolk.
Stir mixture together and refrigerate for at least 30 minutes to allow mixture to firm up.
Using a melon baller or teaspoon, clean out the cavity of each mushroom.
Fill each mushroom with 1 Tablespoon filling or more creating a sizable mound over the top.
Top with remaining Parmesan cheese.
Place mushrooms filling side up on the baking tray.
Bake in the preheated oven until the mushrooms are tender and the topping is lightly browned, 20 to 25 minutes.
Garnish with chopped parsley.