Soak grilling papers and twine in water according to pkg. directions. Brush sirloin with 1 tbsp. olive oil; sprinkle with seasoning and set aside. Toss potatoes in a small bowl with remaining olive oil; microwave on high, covered, for 5 min. Preheat grill to high heat. Arrange sirloin on grill rack; grill on both sides until grill marks appear (1 min. per side). Remove from grill; arrange each piece of sirloin on center of each of 4 grilling papers. Divide potatoes and carrots among grill papers on either side of sirloin; fold grill paper in towards sirloin so edges overlap. Tie each bundle with twine to secure. Place on grill rack; grill, covered, for 5 min. per side. Divide contents of grilling papers among 4 dinner plates; top sirloin with Horseradish Chive Sauce and fried onions. Serve immediately.
HORSERADISH CHIVE SAUCE: In a small bowl, combine ½ cup sour cream, ½ cup mayonnaise, 3 tbsp. Kowalski's English-Style Horseradish Sauce and 2 tbsp. minced chives; refrigerate, covered, until ready to use.