Wash the fish well in salt water. Apply the lemon juice to the fish and set aside for approximately 1 hour.
Fry the onions till they are light golden in color. Remove with a slotted spoon and crush. In the same oil put the red chillies, onion seeds, turmeric powder, red chilly powder and garlic. Stir and fry for a few minutes till the oil separates.
Add the tomatoes and again stir and cook till the oil separates. Add the fish. Coarsely grind together the aniseed, coriander seeds, fenugreek seeds and mustard seeds.
Add this to the fish along with the green chillies and cook over a low heat till the fish is done, about 10 minute min.