Put the eggs in a frying pan covered with cold water and let them boil. From boiling point, meanwhile, make hard for 7 min. Quench immediately after cooling.
Rinse the cress, dry it thoroughly in a clean dishcloth, then chop it coarsely with a chopping knife. Whip the cream until stiff. Mix mayo, sour milk and juice of one lemon. Stir in the cream and the cress and season the sauce with salt and pepper.
Cut the slices of smoked salmon in half lengthwise. Peel the eggs and cut them in half. Put some cress sauce on a plate. Arrange the egg halves and slices of smoked salmon decoratively on top and pour the remaining sauce over them.
Serve with toast or crusty Parisian bread.