Pat the ribeye steaks dry with paper towels. Season both sides generously with salt and freshly ground black pepper.

In a large skillet, heat olive oil over medium-high heat. When the oil is hot, add the ribeye steaks and sear for about 4-5 minutes on each side until a deep brown crust forms. Remove the steaks from the skillet and set aside.

In the same skillet, add the halved shallots and sauté for about 2-3 minutes until they begin to caramelize. Add the minced garlic and cook for an additional minute until fragrant.

Pour in the sherry, scraping up any brown bits from the bottom of the skillet. Bring to a simmer and cook for about 5 minutes to reduce slightly.

Add the beef stock and fresh thyme leaves to the skillet. Return the ribeye steaks to the skillet, spooning some of the shallots and liquid over the top. Cover and reduce the heat to low. Simmer for about 30-40 minutes for medium-rare, or longer for your desired doneness.

While the steaks are braising, bring a pot of salted water to a boil. Add the diced celeriac and cook for about 15-20 minutes until tender. Drain and return to the pot.

Add the roasted bone marrow to the cooked celeriac and mash together until smooth. Incorporate the unsalted butter and season with salt and pepper to taste.

Once the ribeye is cooked to your liking, remove from the skillet and let rest for a few minutes. Slice the steaks against the grain. Plate the celeriac and bone marrow puree, top with sliced ribeye, and drizzle with the sherry-braised shallots and sauce. Garnish with fresh thyme sprigs.