Bring a large pot of lightly salted water to a boil. Add the elbow macaroni and cook until al dente, about 8-10 minutes. Drain and rinse under cold water until cooled.
In a large bowl, combine the cooled macaroni, tuna, celery, red onion, and hard-boiled eggs. In a small bowl, whisk together the mayonnaise, salt, Dijon mustard, and black pepper.
Pour the dressing over the salad and gently toss to coat evenly. Cover the bowl and refrigerate for at least 1 hour to let the flavors blend. Adjust seasonings as needed. Garnish with parsley when serving.