Place beans in a Dutch oven and cover with water; bring to a boil. Cook, uncovered, for 8-10 minutes or until crisp-tender; drain and set aside.
In a small bowl, combine the sugar, cornstarch, vinegar, water, mustard and salt until smooth. In the Dutch oven, melt butter over medium heat. Stir in the mustard mixture. Bring to a boil; cook and stir for 1-2 minutes or until thickened. Return beans to the pan; heat through. Serves 12.