Prepare the aromatics: trim and lightly smash the outer layers of the Lemongrass and cut into 2–3 inch pieces; thinly slice the Fresh galangal; tear the Makrut lime leaves to release oils; lightly crush the Thai bird's eye chilies (leave whole for mild heat or slice for more); and halve or thinly slice the Shallots.
In a medium pot, bring the Chicken stock to a gentle simmer and add the prepared lemongrass, galangal, lime leaves, chilies, and shallots. Simmer uncovered for 8–10 minutes to infuse the broth with the aromatics.
While the broth infuses, thinly slice the Boneless skinless chicken thighs against the grain into bite-sized pieces for tender, quick cooking.
Add the sliced chicken to the simmering, aromatized stock and cook until the chicken is just about done, about 4–6 minutes.
Stir in the Straw mushrooms and simmer another 2–3 minutes until they are heated through.
Lower the heat and gently stir in the Coconut milk, warming it through without bringing the soup to a hard boil (to prevent the coconut from breaking).
Season the soup: add Fish sauce to taste for saltiness and a small amount of Palm sugar to balance acidity and heat—start with a teaspoon and adjust as needed.
Finish by removing the pot from heat and stirring in the Fresh lime juice to brighten the flavors. Taste and adjust fish sauce, sugar, or lime as desired.
Ladle into bowls and garnish with chopped Fresh cilantro (coriander) and thinly sliced Spring onions. Serve hot, with extra chilies or lime wedges on the side if you like additional heat or acidity.
