Make slits along the length of the fish, on one side, going deep into the fish, along the bone, but not cutting through the opposite side-- thus creating a pocket. Repeat on the other side of the bone, and you will have two such pockets on either side of the bone of the fish.
Apply the salt and the lemon juice to the fish inside out, leave for about 15 minutes and wash off. Drain and wipe dry.
Apply the ground paste all over the fish, inside too. Heat the oil to high, and add the fish to it, browning over high heat, first on one side then the other.
Lower the heat, cover and cook till cooked through (10-15 minutes, depending on the thickness of the fish.)
Transfer on to a serving dish, garnish with the lemon juice and chaat masala and serve.