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