1.Preheat oven to 350 degrees F. Combine sausage, 1 cup shredded cheese, ricotta cheese, onions and tomatoes in large bowl.
2. Brush tops of mushroom caps with oil; place, top-sides down, in shallow baking pan. Top with sausage mixture. Drizzle with vinegar, if desired. Sprinkle with remaining shredded cheese.
3. Bake 10-12 minutes or until mushrooms are tender.