In a large skillet, cook bacon over medium heat until crisp. Blot on paper towels. Drain drippings, reserving 2 tablespoons. Crumble bacon and set aside. Cut onions lengthwise into 1/2” wedges. In the drippings, cook onions over medium-low heat until tender and golden brown, about 50 minutes.
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 vinegar and brown sugar into onions; add beans.
Cook, uncovered, on medium heat for 1 minute. Add bacon, toss gently. Season with salt and pepper.
Yields 9 servings