Bring a saucepan of water to the boil and cook the matchsticks of potato for 1 minute and then drain and leave to cool.
Place a large frying pan or wok over a high heat. Add 1tbsp of the oil to the pan and quickly cook the garlic, chili, ginger, red pepper and red onion for 2-3 minutes until they have a little colour and soften. Remove from the pan then heat the remaining oil the in wok until hot and cook the beef until nicely coloured.
Now add the teriyaki sauce, water and potatoes and bring to a boil, stirring as you go. Tip in the vegetables along with the pak choy and spring onions, cook for 1 minute and serve sprinkled with some toasted sesame seeds if you have any.