Filet Of Beef With Potato Gratin, King Trumpet Mushrooms, And Bone Marrow Bearnaise Recipe

Prepare and assemble

Preheat oven to 375°F (190°C). Thinly slice potatoes 1/8 inch (use a mandoline for even slices). In a small saucepan, warm heavy cream, milk, minced garlic, nutmeg, salt and pepper until just below simmering; remove from heat.

Layer and bake

Butter a shallow baking dish (about 8x8 or oval gratin). Arrange a single overlapping layer of potato slices, sprinkle a little grated Gruyère and Parmesan, then add more potato slices. Repeat until all potatoes are used, finishing with cheese on top. Pour warmed cream mixture evenly over the potatoes so it seeps through the layers. Dot surface with the 1 tablespoon butter. Cover tightly with foil and bake 35 minutes. Remove foil and bake an additional 20–25 minutes until top is golden and potatoes are tender when pierced. Let rest 10 minutes before serving.

Make tarragon reduction

Combine white wine vinegar, dry white wine, minced shallot and half of the chopped tarragon in a small saucepan. Bring to a simmer and reduce over medium heat until about 2 tablespoons of liquid remain (about 5–7 minutes). Strain solids and cool slightly.

Emulsify egg yolks

Place the egg yolks in a heatproof bowl set over (but not touching) simmering water (double-boiler). Whisk the reduced vinegar mixture into the yolks continuously until the mixture thickens and becomes pale and ribbon-like (about 3–4 minutes). Keep bowl warm but remove from heat if it gets too hot.

Incorporate butter and marrow

Slowly drizzle in warm clarified butter while whisking constantly to form an emulsion. After about half the butter is incorporated, whisk in the roasted bone marrow (soft spooned marrow) so its rich flavor blends into the sauce. If the sauce becomes too thick, add 1 teaspoon of warm water to adjust consistency. Season with salt and white pepper/cayenne. Stir in remaining chopped tarragon and chervil/parsley. Keep sauce warm in a thermos or over very low heat (off direct steam) until serving.

Prepare mushrooms

Trim the base of king trumpet mushrooms and slice lengthwise into 1/4-inch thick planks. Pat dry with paper towel.

Sear mushrooms

Heat a large skillet over medium-high heat. Add 1 tablespoon olive oil and 1 tablespoon butter. When foaming subsides, add mushroom slices in a single layer (work in batches if needed). Sear without moving until golden brown on one side, 2–3 minutes, then flip and brown the other side 2 minutes. Toward the end, add chopped thyme, salt, pepper and lemon juice. Remove mushrooms to a warm plate and keep warm.

Bring to room temperature and season

Remove filets from refrigerator 30–45 minutes before cooking to come to room temperature. Pat dry with paper towels and season all over with 2 tsp salt and 1 tsp black pepper.

Sear and roast

Preheat oven to 425°F (220°C). Heat a heavy ovenproof skillet (cast iron preferred) over high heat until very hot. Add 2 tablespoons olive oil. Sear filets 2–3 minutes per side until a deep brown crust forms. Add 2 tablespoons butter, the crushed garlic cloves and fresh thyme to the pan; tilt pan and spoon the melted butter over the steaks for 30–45 seconds to baste. Transfer skillet to preheated oven and roast until internal temperature reaches 125°F (51°C) for medium-rare, about 6–8 minutes depending on thickness. Remove filets to a resting board and tent loosely with foil for 6–8 minutes so juices redistribute.

Spoon a generous portion of warm potato gratin onto each plate. Place a rested filet atop or beside the gratin. Arrange 3–4 seared king trumpet slices alongside. Spoon 1½ tablespoons of warm bone marrow béarnaise over each filet (or serve sauce on the side in a small spoon). Sprinkle microgreens or chopped parsley and a pinch of coarse sea salt over the dish. Serve immediately with lemon wedges on the side.