Place the fish pieces in a bowl, add the lemon juice, salt, garlic paste, ginger paste, turmeric powder, Kashmiri red chilli powder and mix.
Heat a pan and dry roast the mustard seeds, fennel seeds, onion seeds and fenugreek seeds. Add the black salt and crush them with a mortar and pestle.
Heat the mustard oil in a pan to smoking point, remove from heat and set aside to cool. Add the yogurt to the fish along with the crushed spices and mix.
Leave it to marinate for thirty minutes. Preheat oven to 200°C/400°F/Gas Mark 6. Add the cooled mustard oil to the marinated fish and mix.
Place the fish on a greased rack in an oven tray. Baste with butter and cook in the preheated oven for ten to fifteen minutes or till done. Serve hot with green chutney.