Heat oven to 350°F. Remove stems from mushroom caps; reserve caps. Finely chop enough stems to measure about 1/2 cup. Discard remaining stems.
In small bowl, mix chopped mushroom stems and remaining ingredients until well blended. Spoon into mushroom caps, mounding slightly. Place in ungreased 15x10x1-inch pan.
Bake 15 to 20 minutes or until hot. Serve warm.