In a large pot, cover potatoes with 1" of salted water. Simmer until potatoes are just tender, approximately 10-12 minutes. Drain and rinse with cold water until cool to the touch. Drain well and chill potatoes until completely cold.
In a small bowl, whisk together mayonnaise, sour cream, white wine vinegar, mustard and salt and pepper. Cover and refrigerate until needed.
Trim bottom off of fennel bulb and pluck off fronds. Cut bulb in half, removing outer layer if needed to clean it up. Cut each half in half again and thinly slice each quarter including stems until you have approximately 1 1/2 cups. Save a few fronds for garnish and coarsely chop remainder.
Pat shrimp dry with a paper towel and combine with potatoes, sliced fennel, fennel fronds and green onions. Add dressing and toss to combine. Chill for at least an hour and adjust salt and pepper to taste. Garnish with fennel fronds and serve.