Wash the meat, dry it and cut it with fibers about 4 cm thick. Wrap with string so that the meat does not lose its shape. Season with salt and pepper. Stir well and let marinate for 1 hour.
“Seal” pieces of meat in a pan with preheated butter. Fry over high heat until a nice crust forms on all sides. Use a pair of tongs or a spatula to turn the meat over, never use a fork, otherwise all the juice will run out. Remove the meat while cooking the sauce in another bowl. Cook the sauce in the same pan.
Wash and chop the mushrooms for the sauce. Fry black pepper in a pan. Add mushrooms and fry for 2 minutes. Dice the onions and add to the mushrooms. Cover, simmer on low heat for about 7 minutes.
Add the cream to the mushrooms and mix well until everything is smooth. Bring the sauce to a boil, salt. Let it cook for about 2-3 minutes, stirring constantly.
Put the meat in the prepared sauce, add oregano and stir. Cover with a lid. Steam for 15 minutes. Switch off the heating and let the bowl cool slightly with the lid closed.
Serve with a warm and plenty of sauce, garnished with vegetable puree or gratin.