Preheat oven to 425 degrees F.
Remove stems from mushrooms. Chop enough stems to make 1 cup. Melt 2 tablespoons margarine in saucepan. Brush mushroom cap tops with margarine, place top side down in shallow baking pan and brush undersides of caps. Heat remaining margarine in same saucepan. Add chopped mushroom stems, onion and garlic and cook until tender. Stir in cream cheese, combining with fork if necessary. Add Parmesan cheese, parsley and herb seasoned stuffing. Mix thoroughly. Spoon about 1 tablespoon stuffing mixture into each mushroom cap. Bake until heated through.
To make ahead, prepare as directed but do not bake. Cover and refrigerate up to 24 hours. Bake as directed.