Mince the mushroom stems. Heat oil in skillet and sauté mushrooms with onions and celery over fairly high heat, stirring often. When vegetables are tender and moisture has evaporated, lower the heat, add herbs and cook a minute longer. Let the 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. May use Swiss cheese in place of Gruyere cheese. Makes 24 servings.