1. Place beans into a large saucepan of boiling water; cook 3 minutes. Drain and rinse with cold water; drain well. Place beans in a large bowl; set aside.
2. Heat 1 1/2 teaspoons butter in a large nonstick skillet over medium-high heat. Add onions; sauté 3 minutes or until lightly browned, stirring frequently. Add broth, vinegar, and sugar; bring to a boil. Simmer 3 minutes or until syrupy. Add beans, remaining 1 1/2 teaspoons butter, salt, and pepper; toss to coat. Cook for 2 minutes or until thoroughly heated.