Bring a large pot of salted water to a rolling boil. Add the shell pasta and cook for 8-10 minutes until al dente, then drain and rinse under cold water.

Finely dice the celery and mince the red onion. Drain and flake the tuna.

In a medium bowl, whisk together the mayonnaise, Greek yogurt, and Dijon mustard until smooth.

In a large bowl, combine the cooled pasta, celery, onion, tuna, and peas. Add half of the dressing and season with salt and pepper.

Cover the salad and refrigerate for at least 30 minutes.

Before serving, stir the salad and mix in the remaining dressing. Adjust seasoning and serve.