Bring a medium pot of water to a boil. Blanche the green beans for 2 minutes. Use tongs or a spider strainer to transfer them directly into an ice bath. Then dry the beans with paper towels.
After rinsing the canned beans, place all of the beans into a large mixing bowl.
In a small mixing bowl, whisk together the Dijon mustard, apple cider vinegar, olive oil, onion, garlic, salt, and pepper until well combined.
Pour the dressing over the beans and toss to coat. Cover the bowl with plastic wrap and refrigerate for 4 to 24 hours.
Allow the bean salad to sit at room temperature for 20-30 minutes to come up to room temperature before eating. Toss the beans in the dressing one more time just before serving.
Notes
If you prefer, you can use 8 ounces of canned green beans instead of fresh. Just drain and rinse them and proceed as directed. I prefer the bright green color of fresh green beans for this recipe.