1. Cut beef in half lengthwise, then crosswise into 1/4-inch slices; set aside.
2. Heat large nonstick skillet over low heat; add onion. Cover; cook, stirring occasionally, 10 minutes or until tender. Remove onion from skillet; set aside.
3. Combine yogurt, sour cream, 2 tablespoons chives, flour, ketchup, mustard, salt and pepper in small bowl; set aside.
4. Heat oil in skillet over medium-high heat. Add beef and mushrooms; cook and stir 3 to 4 minutes or until beef is lightly browned. Return onion to skillet. Reduce heat to low. Stir in yogurt mixture until well blended and slightly thickened, about 2 minutes. Serve over noodles and carrots. Sprinkle with remaining 1 tablespoon chives.