Commercial vegetable broth is readily available in natural-foods stores and many supermarkets. We especially like the Imagine and Pacific brands, sold in convenient aseptic packages that allow you to use small amounts and keep the rest refrigerated.
Heat oil in a large skillet over medium heat. Add onions and cook, stirring occasionally, until golden, 10 to 15 minutes.
Meanwhile, bring a large saucepan of lightly salted water to a boil. Add green beans and cook, uncovered, until crisp-tender, 6 to 7 minutes. Drain.dd broth to the onions; cook for 5 minutes. Stir in vinegar, brown sugar, salt and pepper. Add the beans, cover and cook for 2 minutes. Serve warm.