Dissolve the chicken stock cube in 1 ltr boiling water. Pour into a pan.
Peel and finely slice the ginger. Finely slice the lemon grass. Peel and finely slice the garlic. Finely slice half the chilli (keep the other half for later). Add the ginger, lemon grass, garlic and sliced chilli to the pan. Add the coriander seeds.
Put a lid on the pan. Bring to the boil, then reduce the heat and simmer for 5 mins to flavour the stock.
While the stock simmers, peel the shallots. Finely slice them. Finely slice the last of the chilli.
Carefully pour the stock through a heatproof sieve into a jug or bowl. Pour the flavoured stock back into the pan. Discard the aromatics left in the sieve.
Add the shallots to the pan with the fish pie mix and sliced chilli. Simmer for 2 mins to start cooking the fish.
Add two bundles of the Thai rice noodles to the pan. Simmer for 3 mins. Add the spinach and stir to mix everything together and just wilt the spinach.
Taste the stock and add a pinch of salt if you think it needs it. Ladle the soup into two warm bowls to serve.