Begin by boiling water in a large pot. Once it reaches a rolling boil, add the pasta and cook according to package instructions until al dente. Drain and rinse under cold water to cool.

While the pasta cooks, dice the celery, bell pepper, and red onion. Set aside.

In a large mixing bowl, combine the mayonnaise, Dijon mustard, lemon juice, salt, and pepper. Mix until well blended.

Add the cooled pasta, drained tuna, and diced veggies to the dressing. Gently fold everything together until the pasta is well coated.

Transfer the salad to a serving bowl. Garnish with chopped parsley if desired, and serve chilled. Enjoy!