In a large skillet, cook bacon over medium heat until crisp. Remove to paper towels. Drain, reserving 2 tablespoons drippings. Crumble bacon and set aside. Cook onions in drippings over medium-low heat until tender and golden brown, about 50 minutes.
Meanwhile, place the beans in a large saucepan and cover with water; bring to a boil. Cook, uncovered, for 8-10 minutes or until crisp-tender; drain. Stir the vinegar and brown sugar into onions; add beans. Cook, uncovered, over medium heat for 1 minute. Add bacon; toss gently. Season with salt and pepper. Yield: 9 servings.