In large non-stick frypan, melt butter over high heat. Place flour in pie pan; add chicken and turn to coat well. Place chicken in frypan and cook, turning about 5 minutes to brown well on both sides. Stir in onions, mushrooms and any unused flour. Reduce heat to medium and cook, stirring, until onion is golden brown, about 5 minutes.
In small bowl, whisk together chicken broth and mustard. Pour mixture into frypan and stir. Bring to boil, reduce heat to simmer and cook about 5 minutes. Stir in sour cream and parsley and simmer for 2 additional minutes. Season with salt and pepper to taste and serve. Serve over egg noodles.
To freeze, transfer Chicken Stroganoff to a plastic container with a tight-fitting lid. Let cool, uncovered, for 20 minutes. Refrigerate, uncovered, until cold, about 30 minutes. Cover tightly and freeze until needed.
To thaw, transfer from freezer to refrigerator 12 – 24 hours before needed. Reheat in large, covered frypan over medium-low heat. Bring to simmer and cook about 5 minutes.