Chop the beef, season with salt and pepper, form into a rump steak. Fry the meat only after preparing the potatoes au gratin and the mushroom mousse.
Heat the pan with a small amount of vegetable oil, add diced onions and cleaned whole mushrooms. Season with salt and pepper, fry for approx. 2 minutes. Add butter and mix thoroughly, remove from the heat.
Potatoes au gratin:
Heat oven to 200°C. Grease a 1 L heat-resistant dish with butter.
Layer half the potatoes on the bottom of the dish. Cover the potatoes with a layer of onions, then add the remaining potatoes. Add salt and pepper to taste.
In a medium-sized saucepan, melt butter over medium heat. Add flour and salt and stir for 1 minute with a metal whisk. Add milk and cook the mixture until thickened. Add all of the cheese and cook while stirring, until the cheese melts, about 30 to 60 seconds. Pour the cheese over the potatoes and cover the dish with aluminium foil.
Bake for 1.5 hours in a preheated oven.