Steam the onion rings in margarine for about 5 minutes until they are almost soft. Add the wine vinegar and keep stirring until the liquid has evaporated. Stir the crème fraîche into the onions and simmer for another 5 minutes. Mix in the mustard and season with salt, pepper and a good pinch of sugar.
The sauce can be prepared a day in advance.
Season the pork fillet with salt and pepper. Wrap the bacon around the meat. Place the ham and meat parcels in a greased baking dish. I still fry the meat beforehand. Spread the onion and mustard mixture on top and bake at approx. 220 ° C for 40 - 45 minutes.