Bring the water to a boil, then add the galangal, lemongrass, kaffir lime leaves, onion, and tamarind paste. Boil for about 1 minute, then add the shrimp and mushrooms (devein the shrimp by making a shallow cut along the back and removing the dark vein). Skim off any white foam, then season with fish sauce, salt, and sugar (do not add lime juice yet).

Once seasoned, add the tomato and evaporated milk. Then add the prepared chili oil to taste (stir the chili oil before adding as it may settle). Stir well and taste again, adjusting the seasoning as needed.

Once the soup comes to a boil again, turn off the heat. Squeeze in the lime juice and add the pounded chili peppers, followed by the sliced herbs.