Cut potato in half lengthwise. Cut each half into 3 wedges. Place potato wedges on the unheated rack of a small broiler pan. Brush with 1 teaspoon of the melted margarine or butter. Broil 4 inches from heat for 6 to 8 minutes or until almost brown. Turn; brush wedges with another 1 teaspoon margarine. Sprinkle with garlic salt. Continue broiling for 4 minutes; sprinkle with Parmesan cheese. Broil for 2 to 4 minutes more or until brown. Remove and keep warm.
Place tenderloin on broiler pan. Broil meat for 4 minutes. Turn; broil until meat is of desired doneness (allow 4 to 6 minutes more for medium-rare).
Meanwhile, in a small saucepan combine mushrooms, brandy, garlic, pepper, and remaining melted margarine. Cook over medium heat about 4 minutes or until mushrooms are tender. To serve, arrange tenderloin and potato wedges on a plate and spoon mushroom sauce over meat. Makes 1 serving.