Preheat your oven to 180°C (350°F). Place the beef marrow bone on a small baking sheet and roast for 15 minutes, or until the marrow is rendered and soft. Carefully scoop out the rendered marrow into a small heatproof bowl.

While the marrow roasts, pat the Halal beef tenderloin thoroughly dry and cut it into 2-3 uniform medallions. Season generously with salt on all sides.

Heat a heavy-bottomed skillet over high heat until it is very hot. Add the butter to the skillet and allow it to melt and shimmer.

Carefully place the tenderloin medallions in the hot skillet. Sear for 3-4 minutes per side for medium-rare, ensuring a beautiful, dark crust forms. Reduce heat slightly if the butter starts to burn. Use a meat thermometer for precision (e.g., 55°C). Remove the medallions and let them rest on a warm plate.

Immediately add the rendered bone marrow to the hot skillet (if not using the same skillet, ensure the new pan is hot). Stir rapidly with a whisk or spoon to emulsify the marrow with any pan drippings, creating a rich, glossy sauce. If it's too thick, add a tiny splash of hot water (if allowed on your strict Carnivore interpretation, otherwise ensure enough fat is present).