In a non-stick skillet, heat 2 tbsp of coconut oil and add the mustard seeds. When the seeds start to sputter, add the onions, shallots, curry leaves, chillies, garlic and ginger. Saute until the onions turn translucent.
Mash the onions with a long spoon.
Add turmeric powder, Kashmiri chilli powder, salt, fenugreek powder and fry for a minute.
Add the chopped tomatoes and cook for another minute.
Add the raw mango pieces with 2 cups of water and boil until the gravy is of semi-consistent texture.
Add the fish pieces and cook for another 5 minutes until the gravy becomes thick.
Add coconut milk and let the gravy simmer on low flame for another 5 minutes.
Check the salt and spice levels and adjust accordingly.