Boil cubed potatoes in salted water for 10–15 minutes until fork-tender.
Drain and let potatoes cool completely.
Bring a pot of water to a boil, add green beans, and cook for 3–4 minutes until tender-crisp.
Drain and rinse green beans under cold water.
In a large bowl, mix mayonnaise, sour cream, Dijon mustard, lemon juice, salt, and pepper.
Add potatoes, green beans, red onion, and parsley to the bowl.
Gently toss until everything is evenly coated.
Cover and refrigerate for at least 1 hour before serving.