Slice the green onion diagonally.

Heat salad oil in a frying pan over medium heat. Stir-fry the beef until its color changes, then add the green onion and stir-fry for another minute.

Add sake, sugar, water, and concentrated soy sauce to the pan. Simmer for 1–2 minutes until the liquid reduces slightly.

Boil the diluted soy sauce in a pot, then add the udon noodles and cook according to package instructions.