Using a little water, grind the coriander, chilli and turmeric powders and the garlic into a thick, smooth paste.
In a clay chatti, mix the spice paste, ginger, mango pieces and salt into the cleaned fish. Add a cup of water and cook on a low flame. To stir the curry, gently shake the chatti. A spoon or ladle can break-up the fish pieces. Simmer the curry till the mango has softened and the fish is done. Turn off the flame.
Heat the coconut oil in another pan. Add the mustard and fenugreek seeds and let them crackle, toss in the red chillies, curry leaves and shallots. Fry till the shallots turn golden. Pour the tempering over the fish curry and serve.