Carefully shell some hard-boiled eggs and cut a generous slice off the top of each. Press out the yolk, and rub it into a paste with some finely-chopped ham or chicken, or simply with chopped gherkin. Season with pepper and salt, and add a little warmed butter to mould them. Refill the whites with this mixture, moulding it neatly in the shape of the yolk, and stand each egg on the slice from its end, trimming it with a sharp knife to make it stand upright. Garnish fancifully with slices of gherkin and watercress, the latter seasoned with oil and lemon juice and pepper. If any of the egg mixture remains, form it into small balls and arrange them among the watercress.