Place potatoes in a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, 15 to 20 minutes. Drain, cool, peel, place in a large bowl, and chill.
Meanwhile, combine sour cream, Dijon, dill, parsley, salt, and pepper in a medium bowl.
Pour dressing over potatoes and toss gently. Chill before serving. Top with green onions, if desired.
