The word ‘Klong’ may have derived from a Thai ancient soup recorded as ‘Plong’ which features very similar ingredients. To the untrained eye, this dish may seem identical to Tom Yum, but Tom Klong is cooked with grilled or dried fish rather than fresh fish or prawns and contains tamarind or raw mango rather than lime for its tangy broth.
In the past, villagers were able to catch a huge number of fish during the monsoon season yet had no idea how to store them. Eventually, they came up with ways to preserve the fish by sun-drying them with salt and fermenting in them in fish sauce. These grilled fish were cooked in a soup with dried goat pepper, giving it an aroma distinct from that of the popular Tom Yum. It’s also considered an easier dish to cook than Tom Yum because most of the ingredients or either dried like fish, tamarind, goat pepper and shallots, or staple ingredients in Thai pantries like galangal, lemongrass, kaffir lime leaves and bird’s eye chilis.