Bring a large pot of salted water to a boil. Cook pasta according to package directions until al dente. Drain and rinse under cold water until completely cool. Drain well.

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

Add the cooled pasta, celery, bell pepper, red onion, peas, and parsley (if using) to the dressing bowl. Fold gently with a spatula until everything is evenly coated.

Cover the bowl and refrigerate for at least 30 minutes (or up to overnight) to let flavors meld. Before serving, stir again, garnish with more fresh dill and a grind of black pepper.