Boil eggs and peel them.
Mix well soft butter with mustard, mayonnaise, onion, salt and pepper. Add 9 of the 12 eggs finely chopped to the butter mixture and stir vigorously.
Separate the yolks from the remaining 3 eggs, chop finely and add to the butter mixture as well. Roughly chop the remaining egg whites and add.
Sprinkle with chives.