In a large mixing bowl, combine the medium to large cooked shrimp, diced cucumber, thinly sliced red onion, and freshly chopped dill. Stir gently to ensure the ingredients are mixed thoroughly without breaking apart the shrimp.

In a separate smaller bowl, whisk together sour cream (or Greek yogurt), mayonnaise, freshly squeezed lemon juice, Dijon mustard, salt, and pepper until smooth.

Pour the creamy dressing over the shrimp mixture, and using a spatula, gently toss to coat all the ingredients evenly.

Cover the salad mixture with plastic wrap or a lid and place it in the refrigerator for at least 20 minutes.

Before serving, remove the salad from the refrigerator and give it a gentle stir. Garnish with additional fresh dill or chives.