Prep the Beans. Drain and rinse your canned beans thoroughly under cold water. If using fresh green beans, blanch them in boiling water for 2-3 minutes, then transfer to an ice bath for vibrant color and a tender-crisp texture.
Make the Dressing. In a small bowl or jar, whisk together olive oil, vinegar, sugar, Dijon mustard, salt, and pepper until fully emulsified.
Toss the Salad. In a large bowl, combine the beans, and green onion. Pour the dressing over the top and toss gently to coat.
Chill & Marinate. Cover and refrigerate the salad for at least 1 hour, preferably overnight. This allows the beans to absorb all the delicious flavors.
Serve. Give the salad a good stir before serving, and enjoy chilled or at room temperature.