Slice the green onion diagonally into thin slices and remove the stems of the shiitake mushrooms, slicing them into 5mm pieces.

In a bowl, add egg whites and a pinch of salt, then whisk until stiff peaks form.

Season the beef with salt. Heat salad oil in a pot over medium heat, add the beef, and stir-fry until lightly browned. Add sugar and stir-fry for 1 minute.

Add green onion, shiitake mushrooms, and stir-fry briefly. Pour in the sauce ingredients (sake, mirin, soy sauce, dashi powder, and water). Bring to a boil, skim off any foam, and add the udon noodles. Cook according to package instructions.