From slices are cut rings, lightly fry in a pan greased with butter. Beat the egg yolks, the court placed in a water bath and was added olive oil, stirring continuously until thickening. The mixture is removed from the heat, add the mustard, stirring as not stop, put the lemon juice, pepper, salt and sandwiches are coated, place top slice of ham and a piece of feta cheese, roasted in a hot oven or under the grill. Serve immediately.