1. Line a large ovenproof dish with aluminium foil.
2. Make sure to extend the foil over the edges on all 4 sides.
3. Make 4 to 5 slits in each side of the fish. Place the fish on the foil.
4. Heat oil in a non-stick pan.
5. Add onion, garlic and ginger.
6. Stir and cook until onion is lightly browned.
7. Add cumin seeds, mustard seeds and ground coriander.
8. Stir and cook for 2-3 mins.
9. Remove from heat.
10. Set aside to cool.
11. Puree chopped coriander with the onion mixture and yoghurt.
12. Pour the coriander and yoghurt mixture over the fish.
13. Turn the fish over to coat well on both sides.
14. Cover the fish completely with foil then refrigerate overnight.
15. Next day remove the fish from the refrigerator and leave at room temperature for 30-40 mins. for baking.
16. Cover in foil and bake in moderately hot oven for about 50 mins. or until fish is tender and cooked.
17. In a small frying pan, combine butter, onions and mushrooms.
18. Stir-fry until mushrooms are well browned and slightly crisp.
19. Sprinkle the sauteed mushrooms and extra coriander on the fish.
20. Garnish with lemon wedges and serve hot.

Image courtesy: Flickr/creativecommonsratexla