Prepare the aromatics: bruise the lower stalks of Lemongrass with the back of a knife, thinly slice the Galangal, tear the Kaffir Lime Leaves to release oils, slice the Yellow Onion, halve or quarter the Tomatoes, and finely chop or split the Bird's Eye Chilies (keep seeds for more heat). Soak the Asam Keping in warm water for 10 minutes to soften and reserve the soaking liquid.
Bring Water to a gentle boil in a large pot, then whisk in the Tom Yam Paste until fully dissolved.
Add the prepared lemongrass, galangal, torn kaffir leaves, sliced onion, chopped chilies and the softened asam keping plus a splash (1–2 tbsp) of its soaking liquid. Reduce heat and simmer for 8–10 minutes to let the flavours infuse.
Season the broth with Fish Sauce, Sugar and a pinch of Salt. Stir, then add Lime Juice a little at a time—start modestly and adjust later after the seafood is in.
Gently add the seafood: the Prawns, sliced Squid and chunks of White Fish Fillet. Simmer gently 3–5 minutes, until prawns turn pink and the fish is opaque and cooked through.
Stir in the Straw Mushrooms and pour in the Evaporated Milk to mellow the heat and add body; warm through for another 1–2 minutes without bringing to a hard boil to prevent curdling.
Taste and adjust seasoning—add more fish sauce for saltiness, lime juice for brightness, or sugar for balance. Remove large pieces of lemongrass and galangal if you prefer a smoother broth.
Ladle the soup into bowls, garnish with chopped Cilantro and serve immediately with steamed rice or on its own.
