Mince mushroom stems. Heat oil in skillet and sauté mushroom stems with onions, and celery over high heat, stirring often. Once the  veggies are tender, lower the heat, add herbs and cook a minute longer. Let mixture cool a bit, then stir in the cheese. Preheat oven to 450ºF. Grease a baking pan. Heap filling into mushroom caps and place in pan. Bake 10-15 minutes. Serve hot.