Cook pasta in generously salted water according to package directions until al dente. Drain thoroughly and rinse under cold water to stop the cooking process and prevent sticking.
While the pasta cooks, drain the tuna thoroughly and flake it with a fork. Prepare your vegetables by finely dicing the red onion, chopping the pickles, and dicing the roasted red peppers. If using frozen peas, run them under cold water for a quick thaw.
In a small bowl, combine mayonnaise, mustard, lemon juice, dried dill, garlic powder, salt, and pepper. Whisk until smooth and well combined.
In a large mixing bowl, combine the cooled pasta, flaked tuna, and all prepared vegetables. Pour the dressing over the top and gently fold everything together until evenly coated.
For best flavor development, cover and refrigerate for at least 30 minutes before serving, though it can be enjoyed immediately if needed.