In a small bowl, whisk together the balsamic vinegar, olive oil, garlic, salt, and black pepper. This is your vinaigrette, which will coat the green beans and onions. Adjust seasoning to taste and set aside.

Place the Vidalia onion slices in a large bowl and set aside. The onions will marinate with the green beans, after they are cooked.

Bring a large pot of water to a boil. Add the green beans and cook for about 5 minutes. You want them to be crisp-tender, so don’t overcook them—this will preserve their bright green color and satisfying crunch. Once the beans are done, drain immediately and add them to the bowl with the onions.

Drizzle the warm green beans and onions with the balsamic vinaigrette, tossing to coat everything evenly. The warm beans will help soak up the flavors of the marinade. Sprinkle the salad with parsley and give it another toss.

Now, you have two options: let the salad marinate for 15 minutes on the counter, then serve at room temperature. Or, place it in the refrigerator to chill for 1 hour for a cold salad.