Preheat oven to 400°.
Place 1/2 cup mushrooms in a blender; process until finely ground. Trim fat from tenderloin. Sprinkle beef with ground mushrooms, 1/2 teaspoon salt, and 1/8 teaspoon pepper. Heat oil in large nonstick skillet over medium-high heat until hot. Add beef; cook 1 minute on all sides or until browned. Place tenderloin on a broiler pan coated with cooking spray. Insert meat thermometer into thickest portion of tenderloin. Bake at 400° for 30 minutes or until meat thermometer reaches 145° (medium-rare) to 160° (medium).
Place tenderloin on a platter, and cover with foil. Let stand 10 minutes.
Combine boiling water and 1 cup mushrooms in a bowl; cover and let stand 30 minutes. Drain mushrooms through a cheesecloth-lined sieve into a medium saucepan, reserving soaking liquid; coarsely chop mushrooms. Bring reserved soaking liquid to a boil, and add chopped mushrooms. Reduce heat, and simmer until reduced to 1 cup (about 12 minutes). Add 1/4 teaspoon salt, 1/8 teaspoon pepper, and butter, stirring with a whisk until butter is melted. Serve with tenderloin.