Preheat oven to 400°F. Line a rimmed baking sheet with foil and set aside.
Prepare your mushroom caps by removing the stems (save those stems) and brushing off any dirt that may be on your mushrooms.
Place your mushroom caps onto a foil-lined baking sheet. You want to have your mushrooms with the stem side up so that you can fill them. Set aside.
Chop all your mushroom stems into a fine dice. Set aside.
In a medium skillet, on medium-high heat, add the olive oil, onion, garlic, salt and pepper. Saute for 2 to 3 minutes or until translucent. Add the chopped mushroom stems, spinach and 1 tablespoon of the chopped parsley. Continue to cook for another 3 to 5 minutes or just until the mushrooms are tender. Remove from the heat.
In a large bowl, using a rubber spatula, combine the softened cream cheese and grated parmesan cheese until completely incorporated. Add the cooled mushroom mixture and stir until completely combined.
Transfer your cream cheese and mushroom mixture to a large zip top, or pastry bag. Snip the end off of the corner of your bag and gently squeeze (or pipe) the filling into each mushroom cap. The amount of filling you will use per mushroom cap will vary depending on the size of each mushroom.
In a small bowl combine the seasoned bread crumbs, remaining 2 tablespoons of grated parmesan cheese and 1 tablespoon fresh chopped parsley. Stir to combine.
Sprinkle a small amount of the breadcrumb mixture onto the tops of each filled mushroom.
Bake the stuffed mushrooms for 20 to 25 minutes or until lightly golden.