This roast is everything we love about the holiday. Rosy, incredibly tender and perfectly seasoned meat stuffed with all the quintessential Christmas roast sides -- garlicky spinach, sauteed mushrooms and cubed buttery brioche. Butterflying the roast may seem intimidating but just take your time and the effort will pay off when you hear the accolades at the table. We love how truffle butter adds umami to the dish, but we made it optional – it really is just gilding the lily.
Put the porcini mushrooms in a heatproof bowl and cover with boiling water. Let sit until softened, about 20 minutes. Remove the mushrooms with a slotted spoon so any grit stays at the bottom of the bowl; finely chop the mushrooms.
Melt 4 tablespoons of the butter in a large skillet over medium-high heat; add the cremini and porcini mushrooms. Cook, stirring occasionally, until all the water cooks off and the mushrooms start to brown around the edges, about 10 minutes. Stir in the shallots, thyme, 1/2 teaspoon salt and several grinds of pepper and cook until the shallots soften and the mushrooms turn golden brown, about 5 minutes. Transfer to a baking sheet and spread in a thin layer to cool to room temperature.
Meanwhile, melt 2 more tablespoons of the butter in the same skillet over medium-high heat. Add the garlic and cook, stirring constantly, until fragrant and softened, about 1 minute. Add the spinach and cook, stirring often, until cooked through, about 4 minutes. Season with 1/2 teaspoon salt and stir to combine. Transfer the spinach to a colander set in a bowl to drain any excess moisture. Let cool to room temperature.
While the spinach cools, melt the remaining 4 tablespoons butter in the skillet over medium-high heat. Add the brioche cubes and cook, stirring frequently, until golden brown and crispy, about 10 minutes. Remove from the heat and let cool to room temperature.
Preheat the oven to 450 degrees F.
With a sharp boning knife, cut the meat and fat away from the rib bones about 1 1/2 inches down from the top, scraping the bones as clean as you can. Trim any excess fat so there's no more than 1/4-inch covering the meat. (You can also ask your butcher to do this for you.)
Put the roast on a large cutting board with the bones upright. Holding a large, sharp knife parallel to the bones and about 1 inch from the back of the bones, cut straight down until you are about 1 inch from the bottom of the roast. Gently pull the meat away from the bone so that the bones are upright and the meat is on the cutting board. Position the knife so it is parallel to the cutting board and, starting where you left off with your last cut, continue to cut into the meat about 1 inch up from the board, slowly unrolling the meat as you cut. You will have a long rectangular piece of meat attached to the bone that is about 1-inch thick.
Once the meat is in an even layer, sprinkle with 1 tablespoon salt and 1 teaspoon pepper. Spread the mushroom mixture in an even layer on the meat. Top with the brioche and then layer on the spinach. Dot with the truffle butter if using. Starting at the boneless end, tightly roll the meat back up and secure tightly with butcher twine (tie it all the way around the roast between each rib). Sprinkle the outside with 1 tablespoon salt and 1 teaspoon pepper.
Place bone-side down on a roasting rack set in a roasting pan and transfer to the oven. Roast for 20 minutes, then turn the heat to 350 degrees F and continue to roast until the meat registers 125 degrees F on an instant-read thermometer for medium-rare, about 2 1/2 hours.
Transfer the roast to a cutting board and let rest for 30 minutes before slicing and serving.
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