Clean fish (if used), cut into 5-cm pieces, wash and drain. Shell prawns (if used), remove heads, devein, wash and drain.
Arrange onions, tomatoes, garlic, ginger, green chillies and curry leaves, in the order given, in layers in a pan. Arrange seafood on top. Sprinkle with turmeric, salt and lime juice, and pour in ¾ cup water.
Cover pan, place over moderate heat and bring to boil. Lower heat and simmer for 10 minutes.
Add coconut milk, simmer for 2 minutes longer and remove from heat.
Heat oil for tempering in a small pan and sprinkle in mustard seeds. When they start sputtering, pour contents of pan into moily and mix gently.
