Cut the meat into wafer-thin slices and dust with flour. Leave out 2 tablespoons of margarine in a pan, fry half of the onions and the meat, stirring frequently. Remove the meat and keep it warm.
Leave out the remaining margarine, fry the second half of the onions in it, add the mushrooms to the onions, toast them through. Then add the meat, the white wine and the whipped cream, season with salt and pepper, and simmer for about 5 minutes.