Wash the fish and pat dry. Score the fish (till the main bone) at 1/2 inch intervals on both sides with a sharp knife as shown in the picture.
Marinate the fish with lemon juice and kosher salt. Keep aside for 30 minutes.
Take the lemon grass stalks, ginger, garlic, onions, red chillies and 5-6 gondhoraj lebu leaves, grind to a paste.
Transfer the above paste to a mixing bowl, add the tomato paste, fish sauce, lemon zest, cooking oil, salt , sugar and coarsely ground peanuts. Give it all a hearty mix.
Smear the marinade all over the fish (including the insides) to coat it well. Keep aside for 30 odd minutes.
Cut off the central vein of the banana leaves . Warm the leaves over a very low flame, taking care to not char or burn the leaves.
In a greased baking tray, place two pieces of banana leaf overlapping each other, do ensure they cover the tray. Brush the leaves with oil.
Gently place the fish on the leaves, add the remaining marinade evenly on the top.
Place the remaining gondhoraj lebu leaves on the fish, top with a few lemon slices.
Cover well with another two banana leaves. Brush a little oil on the top of these banana leaves.
Preheat the oven to 200C, bake the fish for 20-25 minutes. Check with a skewer. The skewer should come out clean.
Switch the oven to the grill mode, increase the temperature to 210C. Remove the top banana leaves carefully, grill for 5 odd minutes.
Take the tray out from the oven very carefully and allow the fish to rest for 10 minutes.
Serve hot with a dash of gondhoraj lebu and a bowl of piping hot steamed rice or coconut rice.