1. Heat oven to 400 degrees F. Coat baking sheet with nonstick vegetable-oil cooking spray.
2. Season potato pieces with 1/4 teaspoon salt and 1/8 teaspoon black pepper. Place potatoes on baking sheet. Lightly coat potatoes with cooking spray.
3. Bake potatoes in 400 degree F oven until tender, about 30 minutes, turning over halfway through baking.
4. While potatoes are baking, heat large nonstick skillet over medium heat. Coat with nonstick cooking spray. Season steaks with remaining salt and pepper. Add steaks to skillet; saute about 3 minutes per side for medium-rare or longer for desired doneness. Remove steaks from skillet to platter; keep warm.
5. Add the shallot, mushrooms and garlic to skillet; saute for 5 minutes. Add sherry to skillet; cook for 1 minute. In small bowl, stir together the cornstarch and beef broth until well blended. Add to skillet; cook, stirring occasionally, until sauce is thickened, 2 to 3 minutes. Stir in mustard and parsley. Return steak to skillet; cook until steak is heated through, 1 to 2 minutes.
6. Remove steak to cutting board; slice steak. Serve with potatoes and green beans. Spoon mushroom sauce over steak and potatoes. Makes 4 servings.