Heat coconut oil in a heavy saucepan, add the mustard seeds and fry until they start to pop. Add shallots, garlic ginger paste and chillies. Fry until the shallots are browning.
Add turmeric and masala spice mix and saute two more minutes. Add 1 cup water and salt to taste. When it comes to the boil, add mackerel, mango and curry leaves. Cover and cook over low heat 10 minutes.