Heat the oil in a pan over a medium heat. Add the mustard seeds, cover the pan with a lid and fry the seeds for 20 to 30 seconds, or until the seeds start to pop. Be careful of the popping seeds and the splatter of oil. Once the mustard seeds have started to pop, add the ginger, garlic, onion, and curry leaves, and fry for 2 to 3 minutes, stirring well, or until softened and golden brown.
Reduce the heat to low and add the red chile powder, and ground turmeric. Stir well and fry until the spices are fragrant. Add splashes of boiling water, as necessary, to loosen the gravy mixture, stirring well. Bring the mixture to a boil.
Place the fish in a layer at the bottom of a clay pot or a separate pan, and pour over the curry sauce. Add the fenugreek seeds, salt, and tamarind, and add enough boiling water to just cover the fish.
Bring the mixture to a boil, then reduce the heat, and simmer the mixture for 10 to 15 minutes, or until the sauce has thickened.