Cut the onions into rings and fry them in a little margarine for about 5 minutes. Add wine vinegar and stir until it has evaporated. Stir in the crème fraîche and simmer for another 5 minutes. Stir in the mustard and season with salt, pepper and sugar.
Cut the meat into thin slices, season with salt (a little!) And pepper and wrap with the bacon. Then fry briefly in a pan and then place in a baking dish. Spread the onion and mustard sauce on top. Baked at 200 degrees for 45 minutes.