In a large pot, combine the chicken broth, sliced ginger, bruised lemongrass, and kaffir lime leaves (or lime zest). Bring the mixture to a boil over medium-high heat.
Once boiling, reduce the heat to a simmer and add the bite-sized pieces of chicken breast to the pot. Allow it to cook for about 10 minutes, or until the chicken is cooked through and tender.
Add the sliced mushrooms and thinly sliced onion to the pot. If you’re using Thai chilies or jalapeño for a spicy kick, add those as well. Let everything simmer for an additional 5-7 minutes until the vegetables are tender.
Pour in the full-fat coconut milk and stir to combine. Allow the soup to heat gently without boiling, as you want to keep the creamy texture of the coconut milk.
Add the fish sauce and fresh lime juice to the pot. Taste and adjust the seasoning as needed, adding more fish sauce for saltiness or lime juice for acidity.
Ladle the soup into bowls and garnish with fresh cilantro leaves. Serve hot, and enjoy the incredible aroma wafting through your kitchen!