Put the chili, coriander leaves, mint leaves, curry leaves, pomegranate seeds, and onion in a blender or food processor and process to make a smooth paste. Transfer to a bowl and set aside.
Whisk the hung yoghurt in a bowl, then add all the ingredients for the marinade and the spice paste, season with salt and mix well.
Add the fish cubes and turn until the fish is coated. Cover and allow to marinate in the refrigerator for about 2 hours.
Prepare a tandoor or charcoal grill for moderate heat, or alternatively preheat the grill (broiler) to medium.
Thread the fish cubes onto several metal skewers, 2.5cm / 1-inch apart, and coat with the remaining marinade.
Roast in a moderately hot tandoor, over a charcoal grill or under the grill for 6-8 minutes, or until cooked.
Remove the skewers from the heat and hang them up over a tray for about 2-3 minutes to allow any excess moisture to drain out completely.
Put the skewers in a dish and baste with melted butter then roast again for a further 2-4 minutes.