01 - Bring a large pot of salted water to a rolling boil. Add pasta shells and cook according to package directions until al dente, approximately 8-10 minutes. Drain thoroughly in a colander and rinse under cold running water to halt the cooking process and prevent sticking. Set aside to cool completely.
02 - While pasta cools, finely dice the celery, red bell pepper, red onion, and seeded cucumber. Chop the fresh parsley and dill if using. Ensure shrimp are fully thawed, pat dry with paper towels, and cut into bite-sized pieces if they are large. Combine cooled pasta, shrimp, and all prepared vegetables in a large mixing bowl.
03 - In a separate medium bowl, whisk together mayonnaise, sour cream (or Greek yogurt for a lighter version), Dijon mustard, lemon juice, lemon zest, garlic powder, salt, and black pepper. Whisk vigorously until the mixture is completely smooth and well emulsified, about 1-2 minutes.
04 - Pour the dressing over the pasta and shrimp mixture. Using a large spoon or rubber spatula, gently fold and toss until all ingredients are evenly coated with the creamy dressing. Taste and adjust seasonings with additional salt or pepper as needed. Cover the bowl with plastic wrap or a lid and refrigerate for at least 30 minutes to allow flavors to meld.
05 - Remove from refrigerator and give the salad a quick stir to redistribute the dressing. Serve chilled in individual bowls or on a bed of crisp lettuce leaves. Garnish with additional fresh herbs if desired. This salad tastes best when served cold and can be made up to 24 hours in advance.