Bring a large pot of salted water to a rolling boil, then add the shell pasta. Cook according to the package instructions until al dente, usually about 8-10 minutes. Drain and rinse under cold water.

In a large mixing bowl, combine the diced celery, red bell pepper, red onion, and defrosted peas. Gently fold in the drained tuna, breaking up large chunks while keeping the mixture light.

In a separate bowl, whisk together the Greek yogurt, mayonnaise, dill, lemon juice, Dijon mustard, garlic powder, salt, and pepper until smooth.

Add the cooled pasta to the vegetable and tuna mixture. Pour the dressing over and toss gently to combine.

Cover and refrigerate for 30 to 60 minutes to let the flavors meld.

Once chilled, stir gently and adjust seasoning with more salt or pepper if necessary. Serve as a light meal or side dish.