Make the marinade by mixing together the sugar, salt, soy sauce and cornflour slurry. Mix in the chicken, let marinate for 15 minutes.
Heat a large frying pan with 1 tablespoon oil over high heat. Stir fry the chicken until evenly brown on both sides, 3-4 minutes. Set aside.
Heat the same frying pan with 1 tablespoon oil over high heat. Stir fry the onion and salt until soft, 3-4 minutes. Add the capsicums and stir to mix. Set aside.
Place the vermicelli in the same pan, add onions, capsicums, mushrooms and chicken. Pour over the chicken marinade and water and heat through over high heat, about 2-3 minutes. Adjust the seasoning accordingly and serve hot.
Lee Kum Kee's chicken marinade can be purchased in Chinese/Oriental speciality stores.