Soak the marrow (optional, but helps draw out blood):Place marrow bones in a bowl of cold water with apple cider vinegar. Soak for 30 minutes to 1 hour. Drain and pat dry.
Roast the marrow:Preheat oven to 425°F (220°C). Lay bones on a baking tray, cut side up. Roast for 15–20 minutes until marrow is bubbling and soft, but not completely melted. Let cool slightly.
Make the mousse:Scoop the marrow into a blender or small food processor with the softened butter, rosemary, and sea salt. Blend until smooth and fluffy. You can add a hint of lemon zest or chili flakes for contrast. Set aside or refrigerate.
Prep the steak:Pat the steak dry and season generously with salt and pepper.
Cook the steak: Heat a cast iron pan on high until smoking. Add fat, then sear the steak 2–3 minutes per side (for medium rare), including edges. Let rest at least 5 minutes.
Plate and serve:Slice the steak against the grain, spoon marrow mousse on top (or serve on the side), and garnish with extra rosemary or flaky salt. Optional: a squeeze of lemon cuts through the richness perfectly.
Storage TipsMarrow mousse: Store in an airtight glass jar in the fridge for up to 5 days. Bring to room temp before serving.Steak leftovers: Wrap tightly and refrigerate for 2–3 days. Best reheated gently or enjoyed cold over salad. Estimated Calories (per serving):🥩 Ribeye steak (8 oz / ~225g)~600–650 kcalHigh in protein (~50g) and fat (especially monounsaturated and saturated fats)~120–150 kcalMostly fat (rich in collagen and nutrients)~100 kcal per servingNegligible calories🦴 Bone marrow (from ~1 bone = ~1.5 tbsp usable marrow)🧈 Butter (2 tbsp split between 2 servings)🧂 Extras (rosemary, lemon zest, seasoning)**