Dice the chicken into 1-inch pieces, and set aside. Pour the chicken stock into a large saucepan set over medium heat. Bring the stock to a boil. Add the chicken pieces to the stock and let simmer for 15 minutes. If your stock does not contain salt, add ½ teaspoon.
Cut the lemongrass into 2-inch pieces. Dice the chilies and galangal into thin slices – about 10 pieces. Add the coconut milk to the chicken, along with the lemongrass, galangal, chilies, fish sauce, and Kaffir lime leaves. Be sure to remove the stem from the Kaffir lime leaves and tear the leaves into large pieces. This ensures more flavor is released. Simmer for 5 minutes to allow the broth to infuse.
Dice the mushrooms if they are large; otherwise, use small whole-button mushrooms. Add them to the chicken soup. Simmer for a couple of minutes until cooked.
Dice the scallions and roughly chop the cilantro. Remove from the heat. Add 2 tablespoons of lime juice. Remove the lemongrass and galangal before serving (optional). Add scallions and cilantro to garnish (optional but recommended).