Cut the meat into narrow strips and place in a large enough bowl. Mix with the soy sauce, sake and cornflour and leave for 20 minutes. In the meantime, place the bean sprouts in cold enough water and drain after a few min. Discard the sprouts that do not emerge.
Remove the skin from the onions and slice them. Remove the peel from the ginger and cut into narrow sticks. Cut the leek lengthwise into 4cm strips. Heat 1 tbsp. oil in a frying pan and sauté the onions and ginger. Add the meat and cook for a few minutes, stirring constantly. Take it out with a sieve spoon and put it aside. Clean the frying pan, heat 2 tbsp. oil and sauté the leek and bean sprouts in it until soft. Add the meat, season with salt and cook until well done, bring to the table hot.