This seems complicated, but is actually quite easy to do and although using banana leaf will give the fish more flavour you can also use aluminum foil and still have a delicious, healthy dish. You can choose whichever fish you like, in Kerala they use Pomfret, here I have used Tilapia but use whichever fish is in season and which will work in this dish (your fishmonger will be able to help here). This is not a heavy dish but is really satisfying and just needs a little salad or some potatoes on the side to become a filling, healthy meal. If it is summer, try doing these on the barbeque, the extra smokiness is delicious.
Marinate the fish in the lemon juice with good pinches of salt and turmeric per fillet. Leave aside for 20 minutes.
Heat half the oil in a large frying pan. Add the fish and fry on one side for 20-30 seconds then turn and do the same with this side. Place straight onto the banana leaves or foil packets.
Add the remaining oil to the pan and add the curry leaves, fry for 5 seconds and add the onions, cook well until soft and lightly golden.Add the ginger, garlic and chilli and stir fry for a minute. Add the fennel, clove, black pepper and salt along with the tomatoes and a splash of water. Cook for 5-6 minutes or until the whole thing comes together. Taste, it should taste cooked and harmonious.
Stir the tamarind paste. At this stage it should be a little salty and a little a bit spicy.
Heat the banana leaves over fire to make them pliable and smear the inside of the leaves with some oil. Spoon a little of the masala in the centre of each leaf. Place a piece of fish on top and smear over the rest. If using foil, the masala often sticks to it and you lose a bit so smear a good amount of the mix ontop of one side only. Fold over the foil or banana leaf to enclose the fish (and possible juices) properly and secure using some string or toothpicks to help.
Heat the rest of the oil in a thick-bottomed pan or oil a BBQ rack or griddle pan. Add the parcels and cook, covered, for around 3-4 minutes on each side or until the banana leaf turns a pale brown. If using foil, you can also cook it in a preheated oven (200C) and bake for 10 minutes.
Serve hot with lemon wedges and a salad or some potatoes on the side.