Preheat oven to 400.
Rinse and gently scrub mushrooms. Gently remove stems, finely chop half of the stems, and set aside.
Heat skillet to medium. Add sausage, mushroom stems, onion, and pepper, chopping sausage into small pieces with spoon or spatula.
Saute 10 minutes or until onion is translucent and sausage is lightly browned (stirring, chopping, and mashing sausage to combine with veggies).
Lower heat and add cream cheese in small spoonfuls, mashing and stirring in as cheese melts.
When cheese is fully melted and blended in, remove from heat.
Fill each mushroom cap with sausage mixture and place on oven-safe pan (cookie sheet, casserole dish, or iron skillet). No oil should be needed.
Top with parmesan cheese and parsley flakes, lightly salt, and bake for 30 minutes or until browned, cheese is melty, and juice is flowing underneath mushrooms.