
Fry the onion and garlic in melted butter, then add the mushrooms, salt and fry until golden brown.

Combine the mushroom juice with sour cream, mustard and pepper. Pour the mixture over the mushrooms and simmer everything together for another 2–3 minutes over low heat.
Cut the beef across the grain into 0.5 cm thick slices. Beat lightly, cut into 5–6 cm long strips and sprinkle with flour.

Fry the meat over high heat in a frying pan with well-heated oil for 5 minutes.

Transfer the fried beef to the mushrooms. Heat everything together, but do not bring to a boil. Remove the pan with the finished dish from the heat, cover with a lid and leave for 10-15 minutes in a warm place, then serve.