Place the potatoes in to a pot and fill with enough water to cover. Bring to a boil and cook until tender, about 10 minutes. Bring a separate pot of water to a boil. Add the green beans and cook for about 5 minutes. Drain.
Fry the bacon in a large skillet over medium heat until crisp. Remove bacon and set aside. Fry onions in the bacon drippings until tender and browned. Remove them and set aside. Pour balsamic vinegar into the skillet and stir to release all of the browned bits from the bottom of the pan.
In a large bowl, combine the green beans, potatoes, onions and bacon. Pour the balsamic vinegar over them and toss to coat. Season with salt and pepper.
