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1
For the Stuffing: Process mushrooms in food processor until coarsely chopped, about six 1-second pulses. Heat butter and oil in 12-inch skillet over med-high heat till 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 med and continue to cook, 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.
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2
For the Roast: Season cut side of roast with kosher salt and pepper. Spread cooled stuffing mixture over interior of roast, leaving 1/2-inch border on all sides; lay spinach on top of stuffing. Roll roast lengthwise and tie.
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3
In small bowl, stir together 1 tablespoon olive oil, 11/2 teaspoons kosher salt, and 11/2 teaspoons pepper. Rub roast with oil mixture and let stand at room temperature for 1 hour.
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4
Adjust oven rack to middle position and heat to 450. 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 degrees for rare, 16 to 18 minutes, or 125 degrees for medium-rare, 20 to 22 minutes.
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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.
