Heat the oil in a wok over a moderate heat. Add the steak, garlic and chillies, increase the heat to high and stir-fry for 3–4 minutes or until the steak is browned on all sides.
Pour in the lemon juice and fish sauce, add the sugar and stir-fry until sizzling, then remove the wok from the heat.
Remove the steak from the liquid using a slotted spoon and toss together with the papayas, cucumber, bean sprouts and lettuce. Drizzle the liquid from the wok over the salad ingredients as a dressing and serve hot with a bowl of chilli sauce, if liked.