1. Melt butter in a large nonstick skillet over medium-high heat; add onion. Cook 10 minutes or until golden brown, stirring frequently.
2. While onion cooks, bring cider and broth to a boil in a medium saucepan; boil 23 minutes or until reduced to 1 cup. Combine cornstarch, water, and salt; add to reduced cider mixture, stirring constantly with a whisk. Cook over medium heat 1 minute or until slightly thickened. Stir in onion.