Preheat the oven to 400 degrees F (205 degrees C).
Prepare the Portabella Mushroom Cap (4) by cutting off protruding stems. This will help keep a cavity for the topping. Brush caps with Extra-Virgin Olive Oil (to taste). Bake for 25 minutes.
Meanwhile, prepare the crust topping. In a small bowl, measure out Fried Onions (2/3 cup) and crush into fine pieces with a fork. Add Seasoned Panko Breadcrumbs (2/3 cup), Parmesan Cheese (1/4 cup), and Fresh Parsley (1 teaspoon) to bowl and mix until combined. Set aside.
After baking for 25 minutes, remove the mushrooms and top generously with the topping mixture. Put mushrooms back in oven to bake for an additional 5 minutes.