Rub the fish with the turmeric and a little salt and set aside for 30 minutes.
Meanwhile, toast the cumin and fenugreek seeds lightly (for a minute) in a dry pan and grind together with the other masala ingredients to a fine, creamy paste. Mix the oil leftover from frying the fish, into the masala paste.
Heat the oil in a flat pan on medium heat and lightly fry the fish (2 minutes on each side) and set aside.
Preheat the oven to 200°C and lightly grease the bottom of an oven-proof baking dish.
Place the fish slices in the dish in a single layer and pour the masala over to cover all the pieces.
Cover the dish with aluminium foil and bake for 20 min. Then continue to bake, uncovered for a further 10 min. Serve the fish with fried onions sprinkled on top.