Combine the initial soy sauce (70ml) with the cornflour in a large bowl, and mix until smooth. Stir in the chicken to coat, the put to one side.
Mix the second lot of soy sauce with the vinegar, brown sauce, sesame oil and tomato puree until smooth. Put to one side.
Add 1 tbsp the vegetable oil to the wok over a high heat. Once really hot, add half the chicken. Stir fry until lightly browned and almost cooked through. Lift the chicken out, and repeat with the remaining chicken.
Wipe out the wok, then add in the final 1 tbsp vegetable oil. Quickly stir fry the ginger, garlic and chili, no more than 30 seconds, then add in the peppers, pak choi and spring onions. Stir fry until cooked but firm, then add in the chicken and cashews. Stir fry for a further minute, then add in the sauce and cook until thickened and everything is coated.
Serve alongside a generous portion of rice with plenty of coriander to taste.