Place the stock and water in a large pan over a high heat and bring to the boil.
Add the lemongrass, coriander, ginger, tomatoes, lime leaves, lime juice and red chillies, and return to the boil, then simmer for 15-20 minutes.
Add fish sauce and palm sugar, continuously tasting until you find the right balance of hot and sour to suit your tastes.
Add in the defrosted seafood selection and simmer for a couple of minutes until warmed through. Remove from the heat and serve, garnishing with lime wedges.
Notes
Stir in a can of coconut milk for a smoother Tom Ka recipe, better for little ones who may find this too spicy.