In 3-quart saucepan, heat 4 cups water to boiling. Add beans. Reduce heat to medium. Cover; cook 4 minutes. Add onions. Cover; cook 6 to 8 minutes longer or until beans and onions is desired tenderness.
Meanwhile, in 10-inch nonstick skillet, cook bacon over medium heat, stirring occasionally, until crisp. Remove from heat; stir in mustard, salt and pepper. Drain beans and onions; add to bacon mixture in skillet. Cook and stir until coated and hot. Serve immediately.