Steam the potatoes in their skins until they are tender, about 40 minutes.
Peel and slice them 1/4 inch (6 mm) thick.
Place the slices in a single layer on a baking sheet or one or more platters.
Combine the mustard, egg yolks, vinegar and salt in a food processor and process until blender.
With the machine running, slowly add the oil through the feed tube and process until the mixture is thick.
Stir in the onion and the stock or water and beat until smooth.
Pour the dressing over the potatoes and allow to marinate 1/2 hour.
Sprinkle with parsley before serving.