Bring 6 cups water to a boil in a large saucepan. Add noodles; cook 5 minutes or until al dente. Drain.
Cut beef across the grain into 1/4-inch-wide strips; cut strips into 2-inch pieces.
Heat a large skillet over medium-high heat. Coat pan with cooking spray. Add beef to pan; sauté 4 minutes or until browned. Remove beef from pan. Add 1 cup onion, black pepper, salt, paprika, and mushrooms to pan; sauté 4 minutes or until tender. Reduce heat to medium.
Combine 1/4 cup beef broth and flour in a small bowl, stirring with a whisk. Add broth mixture, beef, remaining 3/4 cup broth, and remaining 1/2 cup water to pan, scraping pan to loosen browned bits. Cover and cook 8 minutes or until sauce thickens. Remove from heat; stir in sour cream, green onions, and butter. Serve beef mixture over egg noodles; sprinkle with parsley.
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