Combine the first 3 ingredients in a bowl. Set aside.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, salt, and pepper in a large zip-top plastic bag. Add beef; seal and shake to coat beef with flour mixture.
Melt butter in a large nonstick skillet over medium-high heat. Add onion to pan; sauté 2 minutes or until tender. Add beef and flour mixture to pan; sauté 3 minutes or until beef is browned. Gradually add broth, scraping pan to loosen browned bits. Add mushrooms; cover and cook 5 minutes or until mushrooms are tender. Reduce heat to low; gradually stir in sour cream mixture. Cook, uncovered, 1 minute or until heated (do not boil). Stir in parsley, if desired. Serve over the egg noodles.