Brown ground beef and onion in large nonstick skillet until meat is thoroughly cooked and onions are tender. Drain excess fat if necessary.
Add mixed vegetables, tomatoes, gravy, marjoram and pepper. Bring to a boil. Simmer over low heat 8 to 10 minutes or until vegetables are tender.
Meanwhile, in medium saucepan, bring hot water, milk and butter to a rapid boil. Remove from heat. Stir in both pouches of potatoes and seasoning just until moistened. Let stand about 1 minute or until liquid is absorbed. Whip with fork until smooth. Serve stew over potatoes.