Drain the sardines and put them in a bowl. Press with the back of a fork until smooth. Add the sieved egg yolks, vinegar, mayonnaise, butter, parsley, and salt and pepper to taste. Blend.
Equip a pastry bag with a star tube (No. 4). Fill the bag with the sardine mixture and pipe it into the egg-white hollows. Garnish the top of each stuffed egg with a small sli