Clean mushrooms with damp paper towels. Remove stems; finely chop 1/3 cup of stems, reserving remaining stems for another use. Set mushroom caps aside.
Coat a small skillet with cooking spray; place over medium-high heat until hot. Add 1/3 cup reserved chopped mushroom stems and celery; saute 2 minutes or until tender.
Combine celery mix, apple and next 5 ingredients in a small bowl; stir well. Spoon 1-1/2 ts apple mix into each reserved mushroom cap. Place mushrooms in a 15x10" jelly roll pan; bake at 350 for 15 minutes.