Bring a large pot of lightly salted water to a boil over high heat. Add green beans and cook, uncovered, until soft but still firm to the bite, 7 to 10 minutes. Check often so beans don't overcook.
Meanwhile, whisk vinegar, olive oil, onion, garlic, parsley, sugar, salt, and pepper together in a large bowl.
Drain green beans and mix with dressing while still warm. Allow to marinate at room temperature for at least 1 hour. Serve at room temperature or chilled.
Easy Green Bean Salad · Dinner Match Lab | Dinner Match Lab