Presoak a clay pot, top and bottom, in water for 15 minutes.
Wash the fish in cold running water, then squeeze the juice of lemon over the fish and let stand while the pot is soaking. Set the juiced lemon aside.
When ready to cook, brush the fish with the melted butter. Slice the reserved juiced lemon and place it both inside and outside of the fish. Place the fish in the pot; a