In a large pot, bring salted water to a boil. Add the baby potatoes and cook for about 15 minutes, or until tender. Drain and let cool slightly.
In the same pot, bring more salted water to a boil. Add the green beans and cook for about 3-4 minutes until bright green and tender-crisp. Drain and immediately transfer to a bowl of ice water to stop the cooking process. Drain again after a few minutes.
In a large mixing bowl, combine the cooled potatoes, green beans, red onion, and parsley.
In a small bowl, whisk together the olive oil, red wine vinegar, Dijon mustard, garlic powder, salt, and pepper.
Pour the dressing over the potato and green bean mixture. Toss gently to combine and coat all ingredients evenly.
Let the salad sit for at least 15 minutes at room temperature to allow the flavors to meld. Serve warm or at room temperature.