Peel onions and cut them into thin rings. Clean, wash and slice the mushrooms. Cut cucumbers into slices as well. Dab meat dry, remove fat edge. Cut meat first into slices, then into thin strips.
Heat the oil in a casserole dish. Brown the meat in it in portions. Season with salt and pepper and remove. Fry onions and mushrooms in hot frying fat for about 5 minutes until the liquid has evaporated.
Dust with flour and sweat briefly. Deglaze with 400 ml water and bring to the boil. Add meat and gherkins, heat up. Stir in sour cream and season everything with mustard, salt, pepper and 1 pinch of sugar.
Rice or boiled potatoes taste good with it.