Place a pot over medium heat and add water, kaffir lime leaves, lemongrass, galangal, seasoning powder, salt, tom yum paste, Thai chili paste, and sliced onion. Bring to a boil.
4 stalks lemongrass, 2 kaffir lime leaves, 45 g tom yum paste, 15 g Thai chili paste, 1/2 onion, 3 g salt, 5 g rosdee seasoning powder, 500 ml water, 10 g galangal