Cut the pork loin steak as a butterfly cut and pound it well. Then peel the onions and cut into thin rings - preferably with a vegetable grater). Season the beaten pork loin steak on all 2 sides with salt and pepper. Finally, coat the inside with mustard and place the onion rings inside. Just close the whole thing and fix it with 2 roulade needles.
Fry the steak in hot oil on both sides until it is golden brown.
I can recommend fried potatoes and a small mixed salad as a side dish.