Clean and possibly wash mushrooms. Cut cucumbers into slices. Peel and halve onions and cut them into thin strips. Dab fillet dry and cut into thin slices.
Heat the lard in a large pan. Fry the fillet slices in it over high heat in portions on each side for 1-2 minutes. Season with salt and pepper. Remove.
Fry the onions and mushrooms in the frying fat until golden brown. Season with salt and pepper. Dust with flour and sweat lightly. Deglaze with wine and 1/8 l water, bring to the boil. Simmer for about 5 minutes while stirring.
Wash the thyme, dab dry and remove the leaves. Stir sour cream, stock and mustard into the sauce and bring to the boil. Heat the meat in it. Season the sauce with salt, pepper and cucumber water. Serve with rice or mashed potatoes.