Remove any tendons and small skins from roast beef. In no case cut away the fat layer. Season thoroughly with salt and season with pepper – using ground pepper if possible. Now sear in a large pan (which also fits in the stove) on all sides. Very important, so the pores close and no gravy comes out.
After searing, put it into the heated stove (220-230ø). 2 kg of roast beef need 35-40 min, then it is nice and pink inside! You measure the core temperature of the meat with a baking thermometer:
So a constant control is inevitable. During the roasting time, constantly baste the meat with the gravy.
For the last 5-7 minutes, brush the roast beef with the butter and herb mixture. With a little top heat or broiler (caution!) a nice herb crust is formed. After roasting, rest the roast beef in aluminum foil for 3-5 min, then cut and bring to the table with the gratin.
Peel potatoes, peel onions and slice narrowly with a vegetable slicer. Brush a gratin dish with butter, layer half of the potatoes in it – season with salt and sprinkle with half of the spice mixture, then put the whole onions on top.
Season again lightly with salt, add remaining potatoes and spice mixture. Pour the whipped cream over the whole gratin. The potatoes and onions must be covered by the whipped cream – perhaps