Slice the lower portion of the lemongrass stalk.
Place chicken broth in a large soup pot over medium-high heat.
Add fresh chicken (or leftover chicken or turkey), corn, chopped onions, carrots and mushrooms. Also add the prepared lemongrass, plus kaffir lime leaves and fresh chili. Boil 5-8 minutes, or until chicken is cooked.
Turn heat down to medium. Add the galangal or ginger, 1/2 can coconut milk, the fish sauce, and extra vegetables (if using). Stir well. Simmer gently 1-2 minutes.
Turn heat down to minimum. Add lime juice and stir.
Do a taste test. Look for a balance between spicy, sour, salty and sweet flavors. Start with salty, adding more fish sauce if not salty or flavorful enough (1 tbsp. at a time). If too sour, add the sugar plus a little more if you need it. If too spicy, or if you’d like it creamier, add more coconut milk. If not spicy enough, add more chili.