Put the chicken stock, galangal, lemongrass, kaffir lime leaves (if using) palm sugar, and Roasted Red Chili Paste (if using) in a medium saucepan and bring to a boil over medium heat.
Add the coconut milk, reduce the heat and bring to a simmer. Add the chicken and mushrooms and continue simmering until the chicken is white and opaque, 5–7 minutes.