Peel onions and cut them into rings. Wash, clean and halve the mushrooms. Wash and quarter the tomatoes. Heat 2 tablespoons of oil in a pan and fry the onions in it.
Add the mushrooms and fry for about 3 minutes. Add tomatoes and braise briefly. Season with salt and pepper. Deglaze with stock and bring to the boil. Add sauce thickener while stirring, bring to the boil again and season to taste.
Wash the meat and dab dry. Heat the remaining oil in a second pan and fry the meat in it over high heat for about 3 minutes on each side. Season with salt and steak pepper. Wash parsley, dab dry and chop finely, except for a little garnish.
Arrange steaks with the sauce on plates. Sprinkle with parsley and serve garnished as desired with a parsley leaf. Serve with baguette with herb butter.