Roast the ingredients under the sections "-- For roasting --" and "-- For grinding to paste --" separately. Cool them to room temperature then grind them into a thick fine paste by adding little water as possible.
In a pan heat sesame oil. Add mustard seeds, fennel seeds, cumin seeds, curry leaf and sauté until the mustard seeds stop splattering.
Now add onion, garlic and sauté until the onion turns translucent (golden brown).
Now add the tomatoes and sauté until they become mushy or until oil separates.
Now add the ground paste, tamarind juice, salt, water as needed and cook until oil separates.
Now add the fish pieces and cook for 5 minutes and serve.