In a deep skillet heat oil and add the Nigella Seeds.
Add the crushed garlic and saute on low heat until golden but not burnt.
Add the mustard paste.Saute for about a minute and add the tomatoes.
Mix well and cook for about a minute and add the ground Coriander, Turmeric,Red Chilli and salt.
Stir frequently on medium heat for about 4-5 mins until all the paste comes together and the tomatoes are cooked.
Add the fish fillets and mix very gently.Add the water and bring to a boil.
Cover and cook on medium until the fish is fully cooked,about 5-6 mins.Add the chopped coriander leaves and turn off the heat.
Sprinkle the Amchur powder or lemon juice and mix.Your fish is ready!Serve hot with rice.