Fill a large pot with water, place over high heat and bring to a boil. Add about 2 tablespoons of salt and the macaroni and stir.
Boil macaroni for about 12-15 minutes or until al dente, stirring occasionally. Drain and set aside.
Place mayonnaise, bonito flakes, vinegar, and mustard (if using) in a small bowl and stir together. Season with salt and pepper.
Fold carrots, celery, onions, and mayonnaise mixture into the bowl of macaroni and stir together until completely combined.
Place mixture in the refrigerator for at least an hour and adjust seasoning before serving.