Soak the lemon grass in hot water for 5 minutes. Put the coconut milk in a saucepan. Add the lemon grass, galangal, ginger root and lime leaves. Bring to the boil and gently boil until the fat of the coconut milk comes to the surface. Add the chicken, mushrooms, lime juice, fish sauce, chillies (to taste) and sugar. Reduce heat and cook slowly for a further 10 minutes until the chicken is cooked through. Cooking slowly should prevent the coconut milk from separating. Before serving, taste and adjust the sour, salt and sweet balance to your liking by adding more lime juice, fish sauce or sugar. Everyone will have their personal favourite combination.