2 Freeze steak for 30 minutes to firm. Cut across the grain into very thin slices.
3 Halve onion lengthwise, and slice each half.
4 Heat 1 tablespoon of the oil in a saucepan over medium-high heat. Add beef and stir-fry until meat is just brown, about 2 minutes. Remove from pan and drain.
5 Heat remaining 1 tablespoon oil in pan over medium-high heat. Add onions and potatoes and stir-fry for 2 minutes until potatoes are well coated with oil.
6 Add Basic Dashi Stock, sake, and sugar and bring to a boil over medium heat. Add soy sauce and beef to pan and cook until potatoes are tender, about 15 minutes.