Marinade the fish and keep 30 minutes. Deep fry in hot oil.
Soak the mustard seeds, poppy seeds and green chillies in a little water for 30 minutes and then blend to make a paste.
Heat oil. Add the mustard poppy seed paste and let cook for 2-3 min. Add the ginger garlic paste, ½ tsp turmeric powder. Once the oil separates add 2 cups water. Add 2 green chillies and then add tomatoes. Season to taste.
Let the gravy thicken a bit.
Add the fried fish and add 1 tbsp freshly chopped cilantro/coriander. Let cook 5 minutes on slow heat. Don't cook much, else the fish will tear.
Recipe by The Novice Housewife at https://novicehousewife.com/2013/03/26/garam-masala-tuesdays-mustard-fish-curry/