Remove shells from hard-boiled eggs and slice in half lengthwise. Carefully remove the yolks, and transfer to a bowl.

Using a fork or pastry blender, mash the yolks. Add mayonnaise, mustard, cayenne pepper, celery, onion, salt, and black pepper to taste.

Spoon filling into a zip-top bag, cut off a corner tip, then pipe into the egg white halves. Garnish with paprika if you like.

Chill and serve.