Peel onions and slice or cut into fine rings. Clean the mushrooms, wash briefly if necessary and cut into thin slices. Cut cucumbers into slices.
Dab meat dry and cut into strips.
Heat 2 tablespoons of oil in a large frying pan. Sauté the meat in 2 portions while turning. Season with salt and pepper and remove.
Heat another 2 tablespoons of oil in the frying fat. Fry the mushrooms and onions for about 5 minutes, turning them over. Dust with flour and briefly sauté lightly. Deglaze with 400 ml water. Bring to the boil and stir in the stock.
Add cucumbers and simmer everything for about 5 minutes.
Stir sour cream and mustard into the mushroom pan. Add fillet strips and heat up briefly, but do not cook any more! Season with salt, pepper and 1 pinch of sugar. Rice tastes good with it.