1. In a medium saucepan over high heat, combine the stock, soy sauce and mirin and bring to the boil.
2. Lower heat and add the chicken and shiitake mushrooms. Simmer 4-5 minutes or until chicken is cooked through.
3. In the meantime, boil noodles per package instructions. Drain and split evenly between four bowls.
4. Ladle hot broth, chicken and vegetables into each bowl.
5. Drizzle in the sesame oil and garnish with scallions. Serve immediately.