In large roasting pan, arrange onions, then top with tenderloin. Brush tenderloin with 1/4 cup Wish-Bone® Balsamic Vinaigrette Dressing. Bake 55 minutes, basting every 10 minutes with 1/4 cup Dressing, until thermometer inserted in center reaches 140°. Remove tenderloin from pan, let stand 10 minutes.
Meanwhile, add remaining 1/4 cup Dressing to roasting pan and cook over medium-high heat, scraping up brown bits from bottom of pan, 3 minutes or until heated through. Serve sauce with tenderloin.