Remove mushroom stems and use them to prepare the Mushroom Stuffing.Chop mushroom stems finely. Place in a clean cloth and squeeze to extract most of the juice. Heat butter and oil in a skillet. Add onions and cook until wilted. Add mushroom stems and mix well. Add lemon juice, salt and pepper to taste. Cook, stirring constantly, until all of the moisture has evaporated.When stuffing has cooled slightly, add sausage meat and parsley. Mix well. Stuff caps with mushroom-sausage mixture. Mix crumbs and cheese and sprinkle over stuffed mushrooms. Dot each with butter.
Grease a shallow baking dish and arrange caps in it, stuffed side up. Bake in a 400 degree oven for about 20 minutes, basting occasionally with the juices that accumulate in the pan. Serve hot or cold.