Cut the fish fillet into portions. Salt and pepper and then toss in flour. Heat the fat in a pan and fry the fillet pieces until crispy. Remove and place on paper towel. Let cool down.
Bring the water with the vinegar, the onions cut into rings and the spices to the boil and allow to cool slightly.
Layer the fillet pieces in large, sealable jars or mason jars and fill up with the stock so that all the pieces of fish are well covered. Close the jars. Let cool down slowly.
Then rest in a cool place for about 14 days and let it steep.