1. Preheat oven to 350 degrees F.
2. Pop out the stem of the mushroom, leaving the cap. Reserve stems for another use or discard.
3. Brush off any dirt on the mushrooms with a damp paper towel.
4. Mix cream cheese, Parmesan cheese, salt and chopped spinach in a bowl.
5. Fill the opening in each mushroom with the cream cheese mixture.
6. Lightly sprinkle Panko breadcrumbs on top of the cream cheese mixture, if using.
7. Place mushrooms on a baking sheet coated with cooking spray.
8. Bake for about 15 minutes. Juices will begin to pool at the bottom of the mushrooms and the top will begin to lightly brown.
9. Remove from oven and serve hot.