Prepare all the ingredients according to the instructions above. Slice the shallots and chilies before smashing them. Set all ingredients to the side and heat a large pot over medium high heat.
Bring chicken broth to a simmer and add galangal root. Simmer for 5 minutes.
Add coconut milk lemongrass, shallots, lime leaves, mushrooms, and chilies. Simmer for 5 minutes.
Stir in shredded chicken, lime juice, fish sauce, and salt and simmer for 2 minutes, until chicken is warmed. Stir in cilantro and remove galangal root, lime leaves, and lemongrass stalks and serve.
The spice level from the dish will come from the Thai/Serrano chilies. Use more/less to your preference. Taste a small piece before adding so you can determine spice level. You can make this into a delicious Tom Yum soup by removing the coconut milk, increasing the chicken broth by 2 cups and increasing the amount of lime juice 2 tablespoons. Garnish with a lime wedge.