Bring a large pot of salted water to a boil and add the green beans. Cook for 4-5 minutes. While beans are cooking, fill a large bowl with ice water. Drain the beans and immediately plunge in the ice bath.
Heat the olive oil in a large skillet and add the onions. Cook until soft and well browned, about 30 minutes. Add the salt and vinegar and stir.
Drain the beans from the ice water and shake to remove as much water as possible. Add to the skillet with the onions and continue cooking until beans are heated through and onions are very well caramelized.