Generously pierce potato with fork; place on microwavable paper towel. Microwave on High 4 to 5 minutes, turning over once, until tender. Cover; let stand covered 5 minutes.
Meanwhile, in 12-inch nonstick skillet, heat oil over high heat. Add beef, mushrooms and onion; sprinkle with garlic powder and pepper. Cook 5 to 7 minutes, stirring frequently, just until beef is browned and vegetables begin to soften.
Stir in soup. Heat to boiling. Cook uncovered over high heat 7 to 10 minutes, stirring frequently, until mixture thickens. Remove from heat; stir in sour cream.
To serve, place 1/2 potato on each serving plate. Top each with half of the beef mixture; sprinkle each with 1 tablespoon parsley.