1. Cut a small piece from the pointed end of each hard-cooked egg. Cut one third off the rounded end of egg, then carefully remove the yolk, reserving egg white pieces. Arrange egg cups on a large jelly-roll pan lined with foil. Coarsely chop yolks and egg white pieces; transfer to a large bowl. Stir in celery, cornichons, mayonnaise, sour cream, mustard, salt and red pepper sauce.
2. For the cucumber cups, with vegetable peeler, remove 1-inch-wide strips lengthwise down cucumber. Cut cucumber crosswise into 3/4-inch-thick pieces. Using the tip of a small spoon or melon baller, remove the cucumber flesh from each cucumber piece, being careful not to cut through the other side. Finely chop the cucumber flesh, drain on paper towels then transfer to bowl with egg filling. Rinse onion in a small strainer under cold water. Drain and stir into egg filling.
3. Transfer cucumber cups to pan with egg cups. Fill a plastic storage bag with egg filling and pipe into egg whites and cucumber cups. Arrange eggs and cucumbers on a serving plate. Sprinkle tops with chives and poppy seeds, if desired.