Peel the onions and garlic, rinse the champignons. Chop the onion, crush the garlic, cut the champignons into large slices. Prepare a frying pan with thick, high walls.
Fry the onion and garlic in melted butter, then add the champignons, add salt and fry them until golden brown.
Combine 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.
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.