Put the chillies and tomatoes into a blender and blend to form a chilli paste. Set aside.
Heat the oil in a large deep frying pan or wok. Add the onions and saute over medium heat until completely soft and lightly browned, about 20 minutes. Add the garlic cloves, chilli paste, kecap manis, kaffir lime, lemongrass and tamarind.
Continue cooking, stirring from time to time, for 30 minutes or until the mixture has become a rich sauce. At this point the sauce can be cooled and frozen in portions.
When ready to serve, add the fish and simmer until the fish is just cooked through, about 6 – 10 minutes depending on the thickness of the fillets.
Remove the lemongrass, garlic and lime leaves. Serve with basmati rice and steamed green vegetables.