Peel the prawns, leaving the tails intact, but do not discard the heads and shells, as they’re used to flavour the broth. Slice a shallow slit along the back of each prawn and remove the intestinal tract. Optional: For presentation, you can leave a few prawns unpeeled to place on top of the soup before serving.
In a large saucepan, add the prawn heads and shells only (not the peeled prawns), along with the garlic, onion, galangal, lemongrass, kaffir lime leaves and chillies. Pour in the chicken stock and water, then place over medium heat and bring the broth to a boil.
Once the soup comes to a boil, reduce the heat to low and cover the saucepan with a lid. Let it simmer gently for about 30 minutes to allow the flavours to develop. Using a large heatproof strainer, strain the broth into another large saucepan and discard the shells, aromatics and any solids.
Add the tomatoes, sugar and fish sauce to the strained broth and return it to medium-high heat. Once the soup reaches a boil, add the crab along with the peeled and any unpeeled prawns and cook for about 1 minute. Add the mussels, fish and squid, then cook for another minute. Reduce the heat to low and continue cooking for a further 5 minutes, or until the seafood is just cooked through.
Remove the saucepan from the heat and stir in the lime juice. Taste the soup and adjust the seasoning if needed by adding more sugar, salt or lime juice. Give it a final stir, then serve topped with fresh coriander.