This popular curry is called "choo chee" in Thailand. There are different variations of the sauce depending on the region. Here's a typical recipe from Southern Thailand.
3 fresh red bird's eye chillies
½ teaspoon white peppercorns
½ teaspoon cumin seeds
½ teaspoon coriander seeds
10 g lemongrass, chopped
10 g galangal, chopped
10 g garlic
10 g shallot
5 g fresh turmeric
10 g fingerroot
1 teaspoon shrimp paste
Pound the ingredients by gradually introducing them into the mortar. The curry paste is ready when it is smooth: you should no longer be able to distinguish one ingredient from another.
A 500 g snapper type fish, scaled and gutted
2 tablespoons tempura flour mixed with 2 pinches salt
Roll the fish in flour and remove excess. Fry at 170°C for about 5 minutes. Drain on kitchen paper.
200 ml coconut cream
4 tablespoons chicken stock
1 tablespoon fish sauce
½ teaspoon coconut sugar
2 kaffir leaves, thinly sliced
½ fresh cayenne pepper, thinly sliced
Bring the coconut cream to the boil and dissolve the curry paste in it. Add the remaining cream, stock, fish sauce, sugar and half of the kaffir leaves. Cook for 5 minutes and then taste to adjust the seasoning.
Arrange the sauce and fried fish in a serving bowl. Garnish with the remaining kaffir leaves and cayenne pepper.
This recipe is relatively spicy. For a mild curry, reduce the number of chillies used (1 to 2 chillies instead of 3).