Peel and cut the potatoes into bite-sized chunks. Place in pot of salted water and boil for 15-20 minutes until fork-tender. Drain and set aside to cool.

Trim ends of the green beans and cut into 1-inch pieces. Boil salted water and blanch green beans for 3-4 minutes until bright green and tender-crisp. Plunge into ice water, then drain.

In a mixing bowl, combine sour cream, mayonnaise, Dijon mustard, salt, pepper, and chopped dill. Whisk until smooth and creamy, adjusting seasoning to taste.

In a large bowl, gently fold cooled potatoes, blanched green beans, and chopped red onion into the dressing until well-coated.

Cover and refrigerate for at least one hour to allow flavors to meld.

After chilling, stir gently and transfer to a serving platter. Garnish with additional fresh dill if desired.