Bring a large pot of water to a boil over high heat and season with a large pinch of salt. Add the green beans and cook for 3 to 4 minutes until tender but still crisp. Transfer to a boil of iced water to cool and then drain and pat dry.
Add the green beans, onions, and walnuts to a large bowl.
In a separate bowl, whisk together the vinegar and olive oil. Season with salt and pepper, to taste.
Pour the vinaigrette over the green beans, onions, and walnuts and toss to combine. Transfer to a platter and top with crumbled feta and freshly cracked black pepper.