Put the fish in a large sauce pan add the water, lemongrass and turmeric
Bring to a boil then simmer for 10 minutes or until the fish is cooked.
Remove the fish from the pan and when cool enough to handle, peel it and flake the fish, being careful to discard any bones. Set aside the flaked flesh.
Drain the stock through a sieve and reserve for the soup.
For the Onion paste
In a mortar and pestle, pound the onion, garlic, chilies, lemongrass and ginger into a paste.
Heat the oil in a saucepan and add the onion paste.
Cook over moderate heat for a few minutes until the paste is soft and begins to caramelize
Add the shrimp paste, mash with a wooden spoon.
Mix in the turmeric and paprika.
Cook for another minute until fragrant, then add the flaked fish.
Put on the lid and cook covered for about 10-15 minutes.
To make the soup, put the soup paste , rice powder, water and reserved fish stock in a large pan.
Bring to a boil while stirring to make sure the rice powder doesn't clump.
Add the banana stem or blossom and simmer for 30 minutes until tender.
Add the fish sauce, and taste for seasoning. Add in lots of black pepper .
To serve,
put a handful of cooked noodles in a bowl, ladle over the soup.
Serve with all the garnishes.