Place beef ribs in a shallow glass dish. Mix mustard, sugar, lemon juice, salt, pepper, garlic. Spread over beef. Top with onions.
Cover and refrigerate for at least 12 hours, turning beef occasionally.
Heat oven to 350 degrees. Cook beef in a covered pan for 1-1/2 hours until beef is tender and almost done.
To Freeze: Freeze in oven ready dish.
To Reheat: Reheat in 350 degree oven from frozen or defrosted until beef is fully cooked and warmed through.