How to Make It
Heat butter in a small skillet over medium. Add shiitakes and shallot; cook, stirring often, until tender and browned, about 6 minutes.
Cut eggs in half lengthwise. Transfer yolks to a small bowl; add mayonnaise, Dijon, and salt, and mash with a fork to combine. Fold in shiitake mixture. Spoon filling evenly into egg halves. Garnish with chervil.