Sauté onion in butter in a large saucepan over medium-high heat 3 minutes, or until browned. Add wine, and cook 3 minutes. Add beef broth; bring to a boil, and cook 5 minutes.
Stir together 1/4 cup water and cornstarch; add to broth mixture, stirring constantly, 1 minute or until mixture thickens. Remove from heat, and serve over tenderloin.