Prepare egg noodles according to pkg. directions. In large skillet over med. high heat, brown ground beef for 4 to 5 minutes, stirring frequently. Add onion and cook for additional 5 minutes, stirring frequently. Stir in 57 sauce, granules and mushrooms. Dissolve flour in 2 C. of water and add to skillet. Continue to cook for 3 to 4 minutes or until slightly thickened and heated through. Stirring frequently. Gently stir in sour cream and serve over hot noodles.