Trim about ¼ inch white from each end. Cut in half crosswise. Scoop yolks into a small bowl.
Add butter, mustard and a pinch of salt to the yolks. Use a fork to mash thoroughly.
Set whites on a platter, open ends up. Sprinkle lightly with salt. Scoop yolk mixture into a pastry bag (or simply use a spoon or knife) and fill whites.
Zest long fine strips of lemon. Top each egg with curls of zest. If you like, fancy things up with a couple of capers or a few grains of caviar. Enjoy.
Leah Eskin is a Tribune Newspapers special contributor. Email her at email@example.com.