Boil the eggs for 6 minutes from cold (turn the water to a simmer once it boils or your eggs might crack). Run them under cold water as soon as they are done to cool them down and stop black rings forming around the yolks.
Mash together with the butter as finely as you can. Add the mayonnaise and season well. Split the rolls, fill with the egg and top each with a spoonful of Onuga.