Place green beans in a large saucepan and cover with water; bring to a boil. Cook uncovered for 8-10 minutes until crisp-tender; drain and set aside.
In a large skillet, cook bacon over medium heat until crisp. Remove to paper towels and drain, reserving 1 Tbsp. drippings. In the same skillet, saute the onion in drippings until tender.
In a small bowl, combine the cornstarch, salt, mustard, and water until smooth. Stir into the skillet with the onion. Bring to a boil and cook and stir until thickened, about 1-2 minutes. Stir in the brown sugar and vinegar.
Add the beans to this mixture and heat through. Sprinkle with the bacon and serve while hot.