Heat margarine in frying pan over medium heat until it melts. Sauté mushrooms and shallots 4 minutes. Pour in brandy and cook 30 seconds or until most of the liquid absorbs. Place mixture in a bowl and reserve.
Pour oil in frying pan and heat over medium heat. Sauté spinach and garlic 30 seconds or until spinach wilts. Strain spinach in a colander, pressing on it to release all liquids. Combine spinach with mushrooms, 1/4 teaspoons salt, and 1/4 teaspoon pepper.
Slice off fat from tenderloin. To butterfly tenderloin, cut lengthwise, slicing to, but not through, other side. Open the halves, laying tenderloin flat. Cut each half lengthwise, slicing to, but not through, other side; open flat. Place tenderloin between layers of plastic wrap, and flatten to an even thickness, using a meat mallet or rolling pin. Season tenderloin with 1/4 teaspoons salt and 1/4 teaspoon of pepper. Spoon spinach/mushroom mixture down the middle of the tenderloin, being sure to leave 1/2 inch on each side. Roll tenderloin beginning at the short end and tie at 2-inch intervals with heavy string. Coat outside of tenderloin with olive oil and season with the rest of the salt and pepper.
Preheat oven to 500 degrees F.
Move tenderloin to a cooking spray coated baking sheet and bake 35 minutes or until a meat thermometer reads at least 130 degrees F. Let tenderloin rest 10 minutes before cutting.