Roast marrow bones: Preheat oven to 350°F / 180°C. Roast 500g marrow bones for 40–45 minutes. Scoop out marrow, reserve, and keep the roasting juices.
Make mashed potatoes: Boil 1kg potatoes in salted water until very tender (20–25 minutes). Drain, mash or rice, mix in 80g butter and 200ml warm milk. Season and keep warm.
Blanch asparagus: Boil 1.5L water + 15g salt. Cook 400g asparagus for 3 minutes, drain and refresh. Reheat later in 20g duck fat or butter.
Cook steak: Temper 800g bone-in steak, pat dry, season. Sear in a very hot pan with 30g fat for 3–4 minutes per side, plus edges. Finish if needed to reach 125–129°F / 52–54°C. Rest 10 minutes.
Make bordelaise sauce: In steak pan, sweat 40g shallots briefly. Add 250ml red wine, thyme and bay leaf; reduce by two thirds. Add 200ml beef stock/demi-glace and roasting juices; reduce to a coating consistency. Stir in reserved bone marrow, then whisk in 40g cold butter off the heat. Season.
Finish asparagus: Warm asparagus in duck fat or butter until hot and glossy.
Serve: Plate mashed potatoes and asparagus. Slice rested steak and arrange on plates. Spoon bone marrow bordelaise sauce generously over the steak and serve immediately.