Authentic Tom Kha Gai (Thai coconut chicken soup)

Ingredients

• 2 cups chicken stock (unsalted, preferably homemade)
• 1 ½ cups coconut milk (see note 1)
• ½ tsp table salt
• 1 lb chicken thigh (boneless skinless, cut 1-inch pieces (see note 2)
• 1 stalk lemongrass (bottom half only, smashed and cut in 2-inch pieces)
• 12 thin slices galangal (see note 3)
• 5 makrut lime leaves (twisted to bruise and torn into big chunks)
• 1-3 Thai chilies (to taste, smashed until broken or cut into large pieces)
• 2 Tbsp fish sauce
• 1 tsp finely chopped palm sugar ( or granulated sugar)
• 5 1/2 oz oyster mushrooms (torn into bite-size pieces)
• 2 ½ Tbsp lime juice
• chopped green onion and/or cilantro (for garnish)
• Jasmine rice (for serving, optional)