Put the heads and shells in the water or stock and bring to a boil. Use a slotted spoon or strainer to remove the shells, then bring the stock back to the boil. Cut away and discard the bottom 1/2 inch (1.2 cm) of the lemongrass, and then cut the remaining white part of the lemongrass into 1 inch (2.5 cm) pieces. (Do not use the top two-thirds green section; use this for another purpose, such as infusing tea.) Add the cut lemongrass to the stock, plus garlic, shallots and galingale.

Meanwhile, cut the tomatoes each into eight sections. Fresh mushrooms should be rinsed and patted dry, while canned (tinned) mushrooms should be drained and rinsed. Pull or cut away the center stems of the kaffir lime leaves and discard; tear the leaves roughly into pieces. Once the stock is again boiling, add the tomatoes and mushrooms to the pot, followed by the chilies, fish sauce to taste, and lime leaves, in that order; simmer gently for 2 minutes. Increase heat and return to a brisk boil, add the shrimp or prawn meat and cook for no more than 1 minute. (If using pre-cooked seafood, it only requires heating through.) Remove from the flame and add the lime juice, then garnish with the cilantro (fresh coriander).

Note: the fibrous flavors in this dish -- kaffir lime leaf, galingale and lemongrass stems -- are not eaten. Just push them aside when eating the soup.