In a bowl, mix together all ingredients for the sauce and set aside. Heat the oil in a large wok over high heat. Once heated add beef strips, pinch of salt and coarse and stir fry until browned. Add whites of the spring onions and red pepper and stir fry for 1-2 minutes. Stir the sauce, add to the wok and cook until thickened. Finely add udon noodles and ¾ of greens parts of spring onion and mix well. Continue to cook until heated through. Garnish with spring onions and sesame seeds and serve immediately.