Bring a large pot of generously salted water to a boil. Cook the pasta shells according to the package directions until al dente. Drain well and set aside to cool slightly.
In a large mixing bowl, add the cooked pasta shells, diced red onion, sliced celery, thawed frozen peas, and drained tuna. Toss to combine.
In a separate bowl, whisk together the mayo, Greek yogurt, fresh dill, minced garlic, dill relish, and Dijon mustard until smooth and well combined.
Add half of the dressing to the pasta mixture and stir to coat evenly. Add more dressing to taste, depending on how creamy you prefer the salad.
Adjust seasoning and veggies to preference. Refrigerate for at least 30 minutes to allow the flavors to meld and the salad to chill. Top with fresh dill and cracked pepper. Serve and enjoy!
Notes
Frozen peas don't need to be cooked before adding them to the salad. Just let them thaw a bit and add them right in.
If using dried dill, cut the amount in half as it is much more potent than fresh.