Mix the cornflour with the salt, pepper and Five Spice.
Cut the chicken into bite-sized chunks and coat in the flour mix - the way I find easiest is to put everything in a sealed reusable food bag or tupperware box and shake gently until the chicken pieces are all evenly coated.
Coat the bottom of a frying pan with a thin layer of groundnut or vegetable oil and put over a high heat.
Fry the chicken pieces until cooked through - about 2-3 minutes each side. You may need to do this in batches, making sure you don't overcrowd the pan.
Remove the chicken with a slotted spoon and set aside.
In a wok or clean frying pan, heat a tsp of groundnut oil over a medium heat and add the chopped onion.
Fry the onion for a few minutes until it starts to soften, then add the pepper, chilli and the white parts of the spring onions.
Sprinkle over the salt, pepper, Five Spice (and chilli flakes if using) and stir fry everything together for about 5 minutes, until the vegetables are just beginning to soften.
Add the chicken to the vegetables and toss together.
Sprinkle with the green parts of the spring onions and serve - ideally with your favourite rice.