Beef tenderloin: Cut the meat into pieces, season and brown in oil, place on a perforated plate or on a roasting rack in the oven and cook for about four hours at 70 °C until the core temperature of 52 °C is reached.
Potato gratin: rinse the potatoes, remove the skin and cut into 3 mm thick slices, soften with the remaining ingredients. Put them in a baking dish, grate cheese over them and cook them in the oven at 220 °C.
Serve with “Bearnaise Sauce” (=> separate recipe).