Remove fat and tendons from roast beef, wash and pat dry. Rub with salt and pepper. Place on the fat pan of the oven, heat clarified butter and pour over the meat.
Roast in the preheated oven (electric cooker: 225 ° C/ gas: level 4) for about 40 minutes. For the crust, peel and chop the onions. Mix with egg yolk, mustard and breadcrumbs and spread on the roast beef after half the frying time.
In the meantime, wash the potatoes thoroughly and cut in half. Fry in hot oil while turning for about 20 minutes. Season with salt and pepper. Wash the herbs and remove the leaves. Wash lemon, dab dry, grate peel and squeeze the juice.
Add peel and herbs to the potatoes and toss briefly. Mix lemon juice and crème fraîche. Cut the roast beef. Serve the potatoes with crème fraiche. Serve garnished with lemon wedges and parsley leaves.