In a small nonstick skillet, saute the mushrooms until tender. Add the garlic; cook for 1 minute. In a small bowl, combine the mushroom mixture, spinach and cheese; set aside.
Cut a lengthwise slit down the center of the tenderloin to within 1/2 in. of bottom. Open tenderloin so it lies flat; cover with plastic wrap. Flatten to 3/4 in. thickness. Remove plastic; sprinkle with 1/4 teaspoon salt and 1/4 teaspoon pepper. Spread stuffing over meat to within 1 in. of edges.
Close tenderloin; tie at 2-in. intervals with kitchen string. Place on a rack in a shallow roasting pan. Sprinkle with remaining salt and pepper.
Bake, uncovered, at 425° for 30 minutes or until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°). Let stand for 5-10 minutes before slicing.