1Heat oven to 350°F. Remove stems from mushrooms; reserve caps. Finely chop half of the stems; discard remaining stems.
2In small bowl, combine chopped mushroom stems, stuffing mix, cheese, butter and onion; mix well. Spoon about 1 teaspoon mushroom mixture into each mushroom cap. Place on ungreased cookie sheet.
3Bake at 350°F. for 10 to 12 minutes or until thoroughly heated. Serve warm.