INGREDIENTS
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1 (6- to 6 1/2-pound) whole beef tenderloin, trimmed of fat, silverskin, and the chain (see Cooks' Notes); or a 5- to 5 1/2-pound well-trimmed tenderloin roast
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Salt
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Freshly ground black pepper
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4 tablespoons olive oil, divided
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1/2 ounce (about 1/2 cup) dried porcini mushrooms, ground to a powder in a blender
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1/4 cup (2 ounces) black-truffle butter, softened
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1/4 cup all-purpose flour
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1 pound crimini (baby bella) mushrooms, trimmed and quartered into wedges
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3 cups beef stock or low-sodium broth, divided
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1/2 cup chopped scallion greens