Bring a pot of salted water to a boil. Add the green beans and blanch for 1 to 2 minutes. The beans should be barely cooked through and still crisp. Prepare a large bowl of ice water while the beans are cooking. Remove the beans from hot water and place into a bath of ice to stop the cooking. Drain.

Place the green beans and red onion in a large bowl. Toss in olive oil to coat. Sprinkle in balsamic vinegar and season with salt and pepper. Topped with chopped walnuts to serve.