1. FOR THE STUFFING: Process mushrooms in food processor until coarsely chopped, about six 1-second pulses. Heat butter and oil in 12-inch nonstick skillet over medium-high heat until foaming subsides. Add onion, table salt, and pepper; cook, stirring occasionally, until onion begins to soften, about 5 minutes. Add mushrooms and cook, stirring occasionally, until all moisture has evaporated, 5 to 7 minutes. Reduce heat to medium and continue to cook, stirring frequently, until vegetables are deeply browned and sticky, about 10 minutes. Stir in garlic and cook until fragrant, 30 seconds. Slowly stir in Madeira and cook, scraping bottom of skillet to loosen any browned bits, until liquid has evaporated, 2 to 3 minutes. Transfer onion-mushroom mixture to plate and cool to room temperature.

2. FOR THE ROAST: Following illustrations 1 and 2, butterfly roast. Season cut side of roast liberally with kosher salt and pepper. Following illustration 3, spread cooled stuffing mixture over interior of roast, leaving 1/2-inch border on all sides; lay spinach on top of stuffing. Following illustrations 4 and 5, roll roast lengthwise and tie.

3. In small bowl, stir together 1 tablespoon olive oil, 1 1/2 teaspoons kosher salt, and 1 1/2 teaspoons pepper. Rub roast with oil mixture and let stand at room temperature for 1 hour.

4. Adjust oven rack to middle position and heat oven to 450°F. Heat remaining 2 tablespoons olive oil in 12-inch skillet over medium-high heat until smoking. Add beef to pan and cook until well browned on all sides, 8 to 10 minutes total. Transfer beef to wire rack set in rimmed baking sheet and place in oven. Roast until instant-read thermometer inserted into thickest part of roast registers 120°F for rare, 16 to 18 minutes, or 125°F for medium-rare, 20 to 22 minutes.

5. FOR THE BUTTER: While meat roasts, combine butter ingredients in small bowl. Transfer tenderloin to cutting board; spread half of butter evenly over top of roast. Loosely tent roast with foil; let rest for 15 minutes. Cut roast between pieces of twine into thick slices. Remove twine and serve with remaining butter passed separately.

Serves 4 to 6