Cut the fillet into fine strips, dice the onions, flaky mushrooms and cucumber into strips. Fry the fillet strips in portions in the hot fat, season and remove. Sear the onions and mushrooms in the frying fat. Dust the flour over it and sauté briefly. Deglaze with broth and bring to the boil. Stir in sour cream and mustard. Add the meat and cucumber slices and season with salt, pepper and sugar. Garnish with parsley.