Mix the egg white, sesame oil, and soy sauce in a medium to large bowl and add the chicken.
Make the spice mix by combining the Chinese five spice, salt and black pepper in a small bowl.
Pour the cornflour/cornstarch into a large bowl and dredge the chicken pieces (no more than 2-3 at a time) so that they have a generous coating of cornflour. Gently shake off the excess and put onto a plate.
Place a wok or large frying pan on a high heat and add around three-quarters of the oil. Add the chicken pieces when the oil is hot and fry for a few minutes turning regularly, until they are golden brown on all sides. Note that you may have to do this in two batches to avoid crowding the pan. (note 4 & 5)
Place the cooked chicken pieces onto a plate or board lined with kitchen paper when done.
Wipe the wok (note 6) or use a clean wok or frying pan.
Add the remaining oil and fry the onion for a couple of minutes and then add the green pepper and chillies.
Turn the heat down to medium when the green pepper and onion start to soften, add the garlic and fry for one more minute.
Add the chicken, spice mix and spring onions/scallions, and toss everything in the pan together until the spice mix is evenly distributed.
Serve immediately with your choice of sides.