In a small skillet, cook beef and onion over medium heat until meat is no longer pink, breaking beef into crumbles; drain. Stir in the tomatoes, taco seasoning and water. Bring to a boil. Add ramen noodles (discard seasoning packet or save for another use). Cook and stir until noodles are tender, 3-5 minutes.
Spoon into serving bowls; sprinkle with cheese and, if desired, tortilla chips.