1In 10-inch nonstick skillet, cook beef and onion over medium heat 8 to 10 minutes, stirring occasionally, until beef is thoroughly cooked; drain. Remove beef from skillet; set aside.
2In skillet, stir sauce mix, water, milk and butter with whisk until blended. Stir in potatoes. Heat to boiling, stirring occasionally; reduce heat. Cover and simmer 20 minutes, stirring occasionally. Stir in green beans. Cover and simmer 10 to 15 minutes longer or until potatoes are tender.
3Stir in cooked ground beef, mushrooms, sour cream and ketchup. Cover; cook over medium heat until thoroughly heated. Garnish with tomatoes.