Heat the oven to 425 degrees F/220 degrees C. Tuck the lemon slices into the belly of the fish. Stir the salt and egg whites together in a bowl. Chop all but 6 mint leaves. Stir the chopped mint through the salt and put the others inside the fish. Put the fish in a baking dish and cover completely with the salt to seal. Bake 25 minutes. Crack the crust off with a knife, moving all the salt away from the surface. Peel back the skin from the fish, and carefully remove the fillets.