Preheat oven to 425F. Remove stems from mushrooms. Chop enough stems to make 1 cup. Melt 2 tbsp margerine in a saucepan. Brush the top of mushroom caps with melted margerine. Place caps, top side down, in a baking pan and brush the bottoms with margerine. Heat remaining margerine in same saucepan. Add onion, garlic powder, chopped mushroom stems and cook until tender. Stir in cream cheese, breaking up with a fork if necessary. Add croutons, parmesan cheese, and parsely, mixing thoroughly. Spoon about 1 tablespoon stuffing mixture into each mushroom cap. Bake until heated through. To make ahead, prepare recipe as above but do not bake. Cover and refrigerate for up to 24 hours, and then bake as directed. Yields 24 appetizers.