I got the idea for this from "Cook Malaysian" by Lee Sook Ching but, as usual, I had to tailor it to what was in our fridge. The peas are definitely not in the original recipe but they were perfect with it.
So, first put together a couple of diced purple onions (shallots would be better but I didn't have any), lots of sliced garlic, and a couple of tablespoons each of: chopped ginger, lemongrass and lime leaves.
This lot goes into a big frying pan with some oil and a chopped, red chilli. Fry these up for a minute or two, until the aromatic smells start to fill the kitchen, then add a can of coconut milk, a tablespoon or two (depending on your tastes) of tamarind paste, one or two of palm sugar (again according to taste) and a big teaspoon of shrimp paste.
Stir together and bring up to a simmer, then lower the heat and leave it to cook for a while, maybe 10-15 minutes. Taste it, I added fish sauce because I love it, but whether you do is up to you.
I shelled a load of peas because I had fresh but frozen would have done just as well. Throw them in the sauce and let them cook almost through before adding whatever white fish you have; I had whiting fillets.
Cook the fish through but take care not to overcook it. Finally stir in a load of coriander, fresh is best but I didn't have any and the frozen kind is really very good. Spoon over a bowl of rice (instructions for perfect rice here) and sprinkle a sliced spring onion on the top.