Make the paste by putting everything in a blender with 75ml water and whizzing it. It should be as smooth as possible.
Heat the oil in a large saucepan and cook the paste until the water has evaporated and you can smell that the paste is cooking (the smell changes as it goes from raw to cooked).
Add the potatoes, 400ml water and the coconut milk. Bring to just under the boil (don’t let it get up to the boil).
Add the fish sauce, lime, sambal oelek and sugar (if you are using it). Cook over a low heat until the potatoes are tender and the liquid has reduced.
Cut the fish into chunks then stir them into the pan. Allow them to poach for 4 minutes, then add the sugarsnap peas and cook for a further 2 minutes. Taste for seasoning. Add the herbs and serve.