Scale and gut the mudfish, then boil in a large pot for 20 minutes.
Add a pinch of salt to the fish.
Cut lemongrass into thin slices and place into the mortar.
Clean and thinly cut galangal, turmeric, fingerroot ginger, chilli peppers, and garlic cloves, and place in the mortar.
Slice kaffir lime leaves into strips and add to the mortar.
Pound the ingredients in the mortar using a pestle until you achieve a paste-like consistency.
Remove the fish from the boiling water and set aside.
Strain the water in which you boiled the fish through a sieve and set the fish broth aside.
Separate the fish meat from the skin and skeleton, ensuring that you discard all bones. Add the fish meat to the mortar with the kreung paste, but discard the skeleton, skin, and head.
Gently pound the fish meat together with the kreung paste until you create an even mixture of all ingredients.
Boil half a can of the coconut milk in a large pot over medium heat.
Add the fish mixture to the boiling coconut milk and mix together.
Place the fish broth into the pot and mix all the ingredients together.
Add prahok to the pot and stir.
Add the other half of the coconut milk can to the pot and mix all the ingredients together.
Add all the remaining ingredients to the pot and stir.
Cook the resulting fish curry for 15 minutes over medium heat.
Cut green onions and add them to the fish curry just before turning the heat off.
Remove the fish curry from the heat.
Boil the rice noodles until soft (usually 10 minutes), then remove from heat and strain.
Place julienned cucumbers, along with herbs and greens of your choice, into large serving bowls (1 per diner).
Distribute the boiled rice noodles among the diners' bowls, placing them on top of the vegetables.
Pour the fish curry over top of the rice noodles.
Sprinkle the sesbania flowers over top of the fish curry.