Put the slices of steak in a bowl and add the soy sauce, 1 teaspoon of the sesame oil, ginger, vinegar, measurement water, salt and cornflour. Stir well to mix and thoroughly coat the steak slices. Cover and leave to marinate in the refrigerator for at least 20 minutes.
Heat the remaining sesame oil in a wok or frying pan over a high heat. Add the garlic and five-spice powder. Stir-fry for 30 seconds until the garlic is just opaque, then add the marinated steak; reserve any remaining marinade. Stir-fry the beef quickly for a couple of minutes until it is browned on the outside yet pink and tender on the inside, working in batches if necessary to avoid 'stewing' the meat. Remove and set aside.
Add the red and green peppers to the wok or frying pan and stir-fry briskly for 2–3 minutes, tossing them in the oil. Return the steak and the reserved marinade to the pan. Stir-fry for 1 minute until the meat is heated through. Transfer to a serving dish and garnish with thin slivers of chilli, if liked. Serve immediately with straight-to-wok noodles or boiled rice.