Peel the onion and cut into thin rings, turn the flake slices in flour.
Heat the oil in a large pan and fry 2 potato pancakes from the batter until crispy on both sides, remove from the pan and degrease a little on kitchen paper. Fry the flönz briefly on both sides in the pan.
Place a potato pancake on a preheated plate, place the Flönz slices on top and brush with the mustard, put the onion rings on top and cover with the second potato pancake.