One day before grilling, prepare the vegetables to pickle by cutting them into fine strips.
Bring mustard seeds, sesame seeds, sesame oil, salt, mirin, rice wine vinegar, sugar and Ponzu soy sauce to the boil in a saucepan and add approx. 250 ml water.
Put the vegetables in an airtight glass, pour in the vinegar water and close the lid. Leave in the fridge overnight.
Put the chicken in a bowl and marinate for at least 2 hours.
Cut the chicken into strips and place them on top of the tortillas. Garnish with spring onion and sesame seeds and then either eat them like a taco or simply roll into a wrap.