Preheat the oven to 160 degrees C. Heat 1 tablespoon of the oil in a wok or frying pan on medium-high heat. Add the chicken in three or four batches, stir-frying each batch for 3 minutes, or until cooked through. Using a slotted spoon, remove the chicken from the wok. Cover and keep warm.
Heat the remaining 1 tablespoon of oil in the wok. Add the celery, onions and garlic. Stir-fry for 2 minutes, or until almost tender.
Whisk the chicken stock in a small bowl with the soy sauce, cornflour and brown sugar. Add the mixture to the wok along with the mixed Chinese vegetables and sliced water chestnuts. Bring to the boil and cook for 1 minute, or until the sauce thickens and the vegetables are heated through. Make a bed of crisp noodles on a heatproof serving dish and warm in the oven.
Return the cooked chicken, along with any juices, to the wok. Bring to the boil and cook for 1 minute, or until heated through. Spoon the chicken mixture over the hot chow mein noodles and serve immediately.