Preheat oven to 350 degrees F.
Wipe mushroom caps clean with a damp paper towel. Thaw spinach in a colander; squeeze out as much moisture as possible. In mixing bowl, combine all ingredients except mushrooms and Parmesan. Mix well. Fill mushroom caps with mixture and place on a cookie sheet. Sprinkle Parmesan on top. Bake for 15 to 20 minutes. Serve warm.