Preheat the oven for roasting (electric cooker: 100 °C/circulating air: 75 °C/gas: not suitable). Dab meat dry. Rub all around with salt. Wash thyme, shake dry. Place herb stalks on the meat side, wrap with kitchen string.
Heat 1 tablespoon of lard in a large frying pan. Brown the meat on all sides over a high heat. Place a roast thermometer between the bones in the thickest part of the meat. Place the meat on a rack and push it into the oven.
Slide a grease pan underneath it. Cook in a hot oven for approx. 3 1⁄2 hours in pink (medium; core temperature 55-60 °C). Use the pan with the frying set for the sauce.
For the sauce, peel onion and carrot. Cut both into small pieces. Heat the pan with the frying set, fry the onion and carrot in it. Dust with flour. Sauté briefly over medium heat while stirring.
Stir in the wine bit by bit. Pour on the stock and simmer for about 5 minutes. Season to taste with salt and pepper. Pour through a fine sieve into a pot.
Wash or peel the potatoes thoroughly. Cook in boiling salted water for about 20 minutes. Drain and let drain. Lightly press the garlic in its skin with a knife blade. Wash sage and shake dry.
Pluck leaves. Heat 1 tbsp. lard in a large pan. Fry half of the potatoes for about 15 minutes. Add 2 cloves of garlic and half of sage, fry for about 10 more minutes, season with sea salt.
Place on a baking tray and keep warm in the oven. Prepare the rest of the potatoes in the same way.
Remove the roast and lift it onto a plate. Cover with aluminium foil and let it rest for about 10 minutes.
Pour the drained gravy on the tray into the sauce. Bring the sauce to the boil again and season to taste if necessary. Cut the roast beef into slices along the bones. Serve with sauce and potatoes.