Cook the elbow macaroni according to package instructions. Drain and rinse under cold water.

In a large mixing bowl, combine the cooked macaroni, drained tuna, diced celery, red bell pepper, and red onion.

In a separate bowl, mix together the mayonnaise, mustard, lemon juice, salt, and black pepper. Pour over the pasta mixture and stir until well combined.

Cover and refrigerate for at least 30 minutes before serving to allow flavors to meld.