Bring 3 cups of water to the boil, add green tea and leave to infuse for 5 minutes. Strain tea, bring back to the boil, then add 2 cups of hot tea to the flour in a large bowl. Bring together to form a dough and knead for about 5-10 minutes until smooth and elastic. Set aside to rest for 30 minutes, wrapped in cling wrap.
For the Filling:
Combine chicken mince with onion, garlic, ginger, soy sauce, sake and vinegar, then add salt and pepper to taste, then mix in herbs.
For the Chilli Oil:
Gently heat oil in a small saucepan, then add chilli, onions, garlic and a pinch of salt. Leave to infuse in the oven for 1 hour.
For the Orange Ginger Vinegar:
Combine ingredients in a bowl and leave to infuse.
To make the Dumplings:
Roll the dough into logs and then cut into 30 g pieces. Roll pieces into 1 mm thick rounds and place ¾ tablespoon of filling in each. Crimp over edges in a gyoza fold. Heat some oil in frying pan and colour the bottom of the dumplings until golden brown. Add boiling water to come up half way on the dumplings and cover with a lid on and steam 2-3 minutes. Remove lid and cook until the water has evaporated.