Melt margarine in a large nonstick skillet over medium-high heat. Add mushrooms and shallots; sauté 4 minutes. Add brandy; cook 30 seconds or until liquid evaporates. Spoon mixture into a large bowl; set aside.
Heat 1 teaspoon oil in skillet over medium heat. Add spinach and garlic; sauté 30 seconds or until spinach wilts. Place spinach mixture in a colander, pressing with the back of a spoon until barely moist. Add spinach mixture, 1/4 teaspoon salt, and 1/4 teaspoon pepper to mushroom mixture; stir well.
Trim fat from tenderloin. To butterfly tenderloin, slice lengthwise, cutting to, but not through, other side. Open the halves, laying tenderloin flat. Slice each half lengthwise, cutting to, but not through, other side; open flat. Place heavy-duty plastic wrap over tenderloin, and flatten to an even thickness, using a meat mallet or rolling pin. Sprinkle 1/4 teaspoon salt and 1/4 teaspoon pepper over tenderloin. Spread spinach mixture down center of tenderloin to within 1/2 inch of sides; fold over 3 or 4 inches of small end. Roll up tenderloin, jelly-roll fashion, starting with short side. Secure at 2-inch intervals with heavy string. Brush 1 teaspoon olive oil over tenderloin, and sprinkle with remaining salt and pepper.
Place tenderloin on a broiler pan coated with cooking spray. Insert meat thermometer into thickest portion of tenderloin. Bake at 500° for 35 minutes or until the thermometer registers 130° (rare) or 160° (medium).
Place tenderloin on a large serving platter, and let stand 10 minutes before slicing. Serve warm or chilled.