Preheat the oven 375°
Melt the butter in a large skillet and sauté the garlic and onion until soft.
Stir in the breadcrumbs until they are the consistency of wet sand.
In a large bowl add the breadcrumb mixture, softened cream cheese, shredded mozzarella cheese, Parmesan cheese, fresh parsley, and season it with salt, pepper and garlic powder. Mix everything together until it’s evenly combined.
Remove the stems from each baby Bella mushroom and using a spoon, carefully scoop of some of the center to make a slightly bigger and deeper hole. Discarded the stems or save them for something else.
Using a spoon, fill the center of each mushroom with the breadcrumb mixture. Place the mushrooms on a parchment paper lined baking sheet that’s been drizzled with olive oil.
Lightly drizzle the top of each mushroom with a little olive oil and sprinkle with Parmesan cheese.
Place them into the oven for about 20 minutes or until the breadcrumbs become golden brown.
Let them cool slightly and garnish with parsley (if desired). Serve warm and enjoy.