In a bowl combine the chicken, mirin and soy. Cover and chill to marinate for a minimum of 30 minutes or overnight.
Heat a teppanyaki hot plate grill, griddle, fry pan or wok. Very lightly oil the onions, peppers and courgettes. Add to the hotplate and cook until they colour, about 5 minutes.
Drain the chicken from the marinade but ensure you keep this aside. Add the chicken to the hot plate and cook, stirring frequently, alongside the vegetables until no longer pink at the centre and the vegetables are slightly charred - about 8 minutes.
Meanwhile, add the reserved marinade to a small saucepan, bring to the boil and then keep warm on lowest setting.
Plate up with boiled rice. Place chicken on one side of the rice, sprinkled with the sesame seeds, and the vegetables on the other. Pour over the marinade and serve.