Preheat the oven to 400 degrees F.
Heat a large oven-proof skillet over medium-high heat. Season the beef all over with salt and a generous amount of pepper. Add the oil to the skillet and heat until shimmering. Add the butter and swirl to melt. Add the beef and sear until mahogany brown on all sides, about 8 minutes total.
Transfer the skillet to the oven. Roast until an instant-read thermometer inserted in the center registers 125 degrees F. for medium-rare, about 25 minutes. Transfer the roast to a cutting board, tent it very loosely with aluminum foil, and let it rest for 10 to 15 minutes.
Slice crosswise, arrange on a platter, and serve with the mushroom ragout.
1 pound mixed mushrooms, such as shiitake, cremini, or white button
2 to 4 tablespoons unsalted butter
1 medium shallot or 1/2 small onion, chopped
1/2 teaspoon kosher salt
Freshly ground black pepper
3 sprigs fresh thyme, leaves stripped
1/2 cup Madeira, vermouth, or white wine
1/3 cup heavy cream
Clean the mushrooms by brushing with a kitchen towel or a brush to remove any loose dirt. Remove the shiitake stems and discard. Trim the dry ends off the cremini and white mushroom stems. Quarter all the mushrooms and put in a bowl.
Heat 2 tablespoons of the butter in a large skillet over medium-high heat. Add the mushrooms and spread them out evenly in the pan, increase the heat to high. Let the mushrooms cook undisturbed until they brown, then shake the pan to turn them over. Add the additional butter along the sides of the pan as the mushrooms cook if the pan seems very dry. Continue to cook until nicely browned, about 5 minutes. Add the shallot and cook until softened, about 2 minutes. Season the mushrooms with the salt and pepper and add the thyme. Pull the pan off the heat and add the Madeira or the wine. Return pan to the heat and scrape up any of the brown bits that cling to the bottom of the pan with a wooden spoon. Add the heavy cream and bring to a boil. Remove from the heat and serve.
Yield: 2 cups