Using a sharp knife, score each fish 4 times on each side through the skin, about 5mm into the flesh. Squeeze over some lemon juice and sprinkle a little salt inside and out, then set aside for 10–15 minutes.
Blend together all the ingredients for the marinade except the carom seeds and chickpea flour. I often add a little extra paprika for a good red colour. Taste and adjust the seasoning; at this stage it should taste a bit too salty and spicy, and that’s fine. Add the carom seeds and chickpea flour. Smear the marinade thickly over both sides of each fish and into the slits. Leave to marinate for 45–60 minutes.
Meanwhile, preheat the barbecue to a medium-high flame. I like to use a hinged fish rack in which I put the fish, but be careful that it doesn’t flatten it too much. (You can also use 2 metal skewers per fish instead.) Oil the grill rack and the fish rack.
Place the fish on the heat and cook for 6–8 minutes on the first side, or until the underside is a lovely golden colour with some areas of charring. If the coals are too hot, rake them away slightly. Turn the fish and cook the underside in the same way. If using skewers, the fish might stick a little, so be careful before turning. The fish is done when golden on both sides; to check if it is done on the inside, you can use a thermometer which should read 60°C when poked into the thickest part of the fish.
Baste both sides with the melted butter as it cooks and chars, adding paprika to the butter for a good colour, if you want. Sprinkle with chaat masala and serve with lemon or lime wedges and coriander.