Cook the fresh green beans in a pot of boiling water for about 6 minutes until just tender. If using frozen green beans, then cook according to package directions.
Plunge into a bowl of ice water to stop the cooking. Drain and set aside.
In a medium frying pan, cook the bacon until crispy, remove and drain on paper towels.
Remove all but 2 Tablespoons bacon fat from the pan. (If there isn't 2 Tablespoons then add olive oil or butter so you have the correct amount.)
Sauté the red onion until translucent in the bacon fat, about 4-5 minutes.
Add the green beans to the red onion mixture and sauté until heated through.
Put the red wine vinegar onto the beans and toss to fully incorporate.
Crumble the bacon and toss with the beans.
Serve warm or at room temperature.