Onions clean, wash and diagonally cut into pieces. Press meat slices a little flatter. Season with salt and pepper. Spread 1/2 teaspoon of mustard on each slice. Roll up and fix with a skewer.
Heat the oil and fry the rolls for approx. 4 minutes while turning. Remove the rolls from the pan. Fry the onions in the frying fat for 1-2 minutes, season with salt and pepper and deglaze with vinegar. Arrange the roulades on plates.