In a large skillet, bring 1 cup water, oil, salt, and pepper to a simmer over medium heat.
Add green beans and onion. Cover, and cook until beans are crisp-tender, 8 to 10 minutes.
Uncover, and continue cooking, stirring frequently, until water has evaporated, beans are tender, and onions are beginning to brown, 5 to 8 minutes more. Stir in vinegar and serve.