Prepare the Base: In a large pot, combine the coconut milk and chicken broth. Add the smashed lemongrass, torn kaffir lime leaves, and sliced galangal. Bring to a gentle simmer over medium heat.
Add the Chicken: Once the broth is fragrant, add the thinly sliced chicken breast. Simmer for 5-7 minutes until the chicken is cooked through.
Add Vegetables and Seasonings: Stir in the mushrooms (if using) and Thai red chilies. Simmer for another 3-5 minutes until the mushrooms are tender.
