Cook the duck confit in the oven, peel the potatoes and slice them thickly. Sauté the onion in a little duck fat. Roll out a rectangular sheet of puff pastry (preferably).
Cut out 2 rectangles slightly larger than the size of the plate, and spread them with mustard. Cover the lower plate with a rectangle of pastry (mustard side up).
Attach the plate to the appliance. Place half of the potatoes on the lower plate, season with salt and pepper. Flake the meat and distribute the onions and chopped parsley.
Add the remaining potatoes, season with salt and pepper. Place the lid on the pie with the remaining pastry rectangle (mustard on the underside), press down firmly with the upper plate in order to seal the edges of the pie. Attach the upper plate to the appliance.
Bake for 8 minutes.