Authentic Tom Kha Gai (Thai coconut chicken soup)

Name: Authentic Tom Kha Gai (Thai coconut chicken soup)
Prep time: 15 min
Cook time: 25 min
Total time: 40 min
Cuisine: Thai
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); 5.5 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)