In medium mixing bowl, combine ground beef, oats, eggs, chopped onion, salt, and pepper. Mix until thoroughly combined.

Shape beef mixture into patties and brown in skillet, turning once.

Transfer beef patties to 5-qt. slow cooker. Top with sliced onion and mushrooms, if desired.

In mixing bowl, whisk together gravy packets and water until well blended. Add to slow cooker.

Cover and cook on HIGH for about 4 hours, or until beef patties are thoroughly cooked.