Cut the fillet into 2-inch pieces and marinate with turmeric and salt. Wash the fenugreek leaves, discard the bottom 2 inches of the stems, and finely chop the rest.
In a stockpot, bring the coconut milk, ginger purée, serrano, and salt to a boil. Turn the heat down, cover the stockpot, and simmer for 15 to 20 minutes or until the carrots are tender. If the mixture appears to get dry, add some water, and if has too much liquid, cook with the cover off.
Gently add the marinated fish pieces and simmer for 2 minutes. Turn the heat off and stir in the chopped fenugreek leaves.
Heat up the coconut oil and add the asafetida, pop the mustard seeds, and add the green onions. Sauté for 3 to 4 minutes on low heat, then drizzle over the fish curry. Serve immediately.