Combine coconut milk and stock in a large saucepan. Heat to medium-high and bring to a boil. Lower heat, add the chicken pieces and simmer until cooked through. Remove chicken with a slotted spoon and set aside. Add 3 lemon grass stalks, 4 lime leaves and the chiles. Remove larger pices of spices with a larger spoon. Return the chicken to the saucepan along with the mushrooms and heat appx. 3 minutes. Turn off the heat and add some Nam Pla (salty cooking sauce found in international section of the market) , lime juice and sugar. Season with salt and pepper.