Make the marinade by mixing together the sugar, salt, fish sauce, lemongrass and cornflour slurry. Mix in the chicken, allow to marinate for 20 minutes.
Blanch the celery in boiling water for 3-4 minutes, drain and set aside.
Heat a large frying pan with 1 tablespoon oil over high heat. Fry the chicken until evenly brown, 3-4 minutes. Add just enough water to cover 1/2 of the chicken. Simmer for 10 minutes or until the chicken is cooked through. Set aside.
Heat the same pan with 1 tablespoon oil over high heat. Stir fry the lemongrass, eschallot, spring onions, garlic and onion until soft, about 3-4 minutes. Add the celery, chicken, fish sauce, sugar and chilli oil, stir well to combine and serve.