Clean, clean and halve the mushrooms. Peel and wash potatoes and cut or slice them into thin slices (approx. 3 mm). Heat 4 tablespoons of oil in a frying pan in portions. Fry the potato slices for about 3 minutes each. Take out potatoes and fry mushrooms in the frying fat for about 3 minutes while turning. Season with salt and pepper
Grate cheese. Whisk cream and cheese, season with salt and pepper. Fill potato slices and mushrooms into 1 greased casserole dish. Pour cream mixture over it. Sprinkle with mustard. Bake in a preheated oven (electric cooker: 225 °C/ convection oven: 200 °C/ gas: see manufacturer) for approx. 15 minutes
Dab meat dry, cut the fat edge several times. Season with salt. Heat 2 tablespoons of oil in a frying pan and fry the steaks for about 5 minutes, turning them over. Remove the steaks, season with pepper and leave to rest. Wash the thyme, shake dry and remove the leaves from the stems. Arrange gratin on the steaks and sprinkle with thyme