Put the quark, feta and cream cheese in a tall bowl, add the spices and puree everything well with the blender. If you don`t have mustard powder, you can crush normal mustard seeds in a mortar (the finer the better). You can also use a disused coffee grinder. But be careful that the mustard flour does not get too warm (max. 30 degrees), otherwise it will be bitter.
Let it steep overnight, then season again to taste.