In same saucepan, melt the 1 tablespoon butter. Add onion. Cook and stir for 2 to 3 minutes over medium heat until onion is tender, but not brown. Add balsamic vinegar. Cook and stir until slightly thickened, about 2 minutes. Add chicken broth; bring to boiling. Reduce heat. Simmer, uncovered, for 15 minutes, stirring occasionally. Whisk in flour/whipping cream mixture. Cook and whisk over medium heat until thickened and bubbly. Cook and whisk for 1 minute more.