1) Heat medium skillet over medium heat, add the butter and when melted add the onions and cook until soft and tender, 20 minutes. When done, add a splash of water and stir in the mustard.
2) Add sauerkraut to a small pot and keep warm over low heat.
3) Combine sour cream, ketchup and relish in a small bowl and reserve.
4) In a large bowl, combine the meat, spices, parsley, vinegar and salt and pepper, to taste. Form 4 patties, thinner at middle and thicker at edges for more even cooking and to prevent burgers from bulging at the middle. Heat the oil in non-stick skillet over medium-high heat. Cook patties 5 minutes on each side. Melt the cheese over the patties the last 2 minutes of cooking, under foil tent.
5) To serve, pile the onions on the bap bottoms and top with a leaf of lettuce, cheese, wurst burger, sauerkraut, and the sour cream sauce. Cover with the tops of the baps and serve.