In a medium bowl, combine mayonnaise, sour cream, lime zest, lime juice, dill, mustard, minced garlic, salt, and pepper. Whisk until smooth and creamy. Set aside.

Bring a pot of water to a rolling boil. Add the shrimp and cook for 2-3 minutes until pink and opaque. Drain and set aside.

Prepare an ice water bath in a large bowl. Transfer cooked shrimp to the bath to halt cooking. Cool for about 3 minutes, then drain thoroughly.

Chop the cooled shrimp into bite-sized pieces.

In a large mixing bowl, combine chopped shrimp, sliced green onion, and diced cucumber. Pour the prepared dressing over and gently stir to coat evenly.

Cover and chill in the refrigerator for at least 30 minutes. Serve chilled, garnished with extra dill or lime wedges.