Lightly salt and steam your fish filets and keep aside.
In a medium sized pot, heat 1 ½ cups water. Once the water starts bubbling, add the tamarind paste and stir it around so that there are no lumps. The add the jaggery and dissolve. Stir the mixture around till it thickens a bit. Remove from heat and keep aside.
In another saucepan or a wok, pour 2 tablespoons of coconut oil. When the oil is hot, add the mustard and methi seeds, then the curry leaves and chopped ginger. Once the ginger browns a bit, add some crushed chilli flakes or chilli powder. Then pour in the tamarind-jaggery liquid and let it simmer for a few minutes.
Next, add the steamed fish fillets and a teaspoon of chopped coriander stems. Let the curry bubble for a few minutes more, so that the fish can absorb the flavours. Throw in a few curry leaves as well.
Turn off the heat, garnish with some cilantro and curry leaves and you’re ready to plate up. Serve with white rice.