Sardines in Eggs

Skin and bone ten or a dozen sardines and place them in a mortar, remove the
shells from an equal number of hard-boiled eggs, cut them in pieces crosswise, put
the yolks in the mortar with the sardines, adding at the same time a little chopped
parsley, salt, pepper and one-tablespoonful of butter; pound all together, fill the
whites of the eggs with the above mixture, stick them together like whole eggs,
arrange them on a dish with watercress between them and serve.