Heat the butter in a frying pan. Reduce the heat, add the onions, stir fry for approx. 10 mins. Add the tomato puree and briefly continue cooking. Pour in the wine, reduce for approx. 1 min. Combine the stock and cornflour, add while stirring, cook for approx. 3 mins., season.
Heat the clarified butter in a non-stick frying pan. Add the roesti, shape into a cake. Continue to cook the roesti (uncovered) over a medium heat for approx. 6 mins. until a golden crust forms. Place a large, flat plate on top of the pan. Tip the roesti out onto the plate. Add a little clarified butter to the pan, slide the roesti back into the pan with the uncooked side facing down, finish cooking for approx. 6 mins. Serve the roesti with the onion sauce.
Heat the oil in a frying pan, fry the bratwurst for approx. 10 mins., Serve with roestis and onion sauce.