Bring a pot of salted water to a boil and cook the pasta according to package directions until al dente. Drain well and spread on a plate or baking sheet to cool to room temperature. This prevents the pasta from becoming mushy when mixed with the creamy dressing and allows it to absorb flavors better.

In a medium bowl, whisk together the sour cream, mayonnaise, mustard, lemon juice, salt, pepper, and celery salt until smooth and well combined. I like to let this dressing sit for a few minutes while I prep the vegetables—it allows the flavors to meld together beautifully.

While the dressing sits, dice the cucumber, radishes, red pepper, and red onion into bite-sized pieces. Chop the gherkins into quarters. Drain the tuna and add it to a large mixing bowl with all the prepped vegetables and gherkins. Pour in the dressing from Step 2 and gently fold everything together until evenly coated. I find that folding rather than stirring vigorously helps keep the tuna from breaking into tiny flakes.

Add the cooled pasta from Step 1 to the tuna and vegetable mixture and gently fold until everything is evenly combined and coated with dressing. Sprinkle the fresh dill over the top and fold in one final time to distribute the herb throughout. Taste and adjust seasoning if needed.

Transfer the tuna pasta salad to a serving dish or individual bowls. Serve immediately at room temperature or refrigerate for up to 2 hours before serving.