Make the marinade by mixing together the sugar, salt, soy sauce and cornflour slurry. Mix in the chicken, allow to marinate for 15 minutes.
In a large frying pan, heat 3 tablespoons oil over high heat. Fry the potatoes until browned, 4-5 minutes. Transfer to a large saucepan.
In the same frying pan, heat 1 tablespoon oil over high heat. Stir-fry the onion and salt until soft, about 3-4 minutes. Add the green pepper and cook for 1-2 minutes. Transfer to the same large saucepan with the potatoes.
Add half of the water into the pan and boil for 5 minutes. Turn the heat down to medium, add the soup and remaining water, mix well, cover and simmer for 15-20 minutes. Turn off the heat and add steamed milk, if using. Mix well and adjust the seasoning accordingly.