Preheat the roast beef oven (electric cooker: 80 °C/circulating air and gas: see manufacturer). Wash meat and pat dry. Cut off the upper layer of fat (some fat should remain). Season with salt and pepper.
Heat the lard in a flat roasting pan. Fry the meat in it over a high heat for 6-8 minutes. With the layer of fat facing upwards, place the meat in the roaster and roast for approx. 4 1⁄2 hours.
For the crust, roughly crush peppercorns and mustard in a mortar. Wash thyme, shake dry and pluck off leaves. Mix everything and after about 1 hour roasting time spread it on the roast beef.
For the potato salad, wash the potatoes and boil them in boiling water for about 20 minutes. Drain, quench, peel and let it cool down.
Onions peel and chop finely. Heat the oil in a wide saucepan. Fry the onions in it until transparent. Add 200 ml water and vinegar, bring to the boil. Stir in stock and simmer for about 2 minutes. Season with salt and pepper.
Cut the potatoes into slices directly into it. Mix everything loosely. Leave to stand for 1 hour.
Peel the cucumber and slice or slice it thinly. Clean, wash and dry lamb's lettuce. Shortly before serving, add the cucumber and lamb's lettuce to the potatoes. Season again with salt and pepper.
Remove the roast beef and let it rest for a short time. Then cut into thin slices and serve with the potato salad. Remoulade tastes good with it.