Wash and dry the blue shark slices with absorbent kitchen paper. Put the oil in a pan with the chopped onion, brown, then add the peas and parsley, mix a few moments, add the tomato purée and the water needed to cover the peas, add salt and pepper. Almost when cooked for the peas, add the slices of blue shark, season lightly with salt and boil over moderate heat, sprinkling the fish with its cooking juices from time to time. After a few minutes remove the blue shark slices, place them on a serving dish and cover with the well-drained peas; reduce the cooking juices further and pour it over the whole.