Preheat oven to 400 degrees F.
Clean mushrooms, removing any traces of soil. Pop the stems from the caps.
Chop the stems and onion really fine, mince garlic. Heat a skillet over medium heat.
Saute stems with onion and garlic for a few minutes until tender.
Combine cream cheese, Parmesan cheese, stem and onion mixture, salt and pepper in a medium bowl.
Chop pepper and spinach; add to cheese mixture and combine.
Stuff the mushrooms caps with the filling and reserve a portion for dipping. Put it in an oven safe container because you are going to bake it along side of the mushrooms.
Bake for 12-14 minutes along with the remaining cheese mixture until the bubbly and golden on top. Serve warm.