Bring a large pot of salted water to a boil. Add the shell pasta and cook until al dente according to the package directions. Drain the pasta in a colander and rinse under cold running water until it reaches room temperature.

Place the cooled pasta in a large mixing bowl. Add the flaked tuna and sliced celery.

In a small bowl or jar, combine the olive oil, Dijon mustard, apple cider vinegar, salt, and black pepper. Whisk or shake until the mixture looks smooth and slightly thickened.

Pour the vinaigrette over the pasta mixture. Toss gently until every shell is coated and the tuna and celery are evenly distributed.

Sprinkle the chopped parsley over the salad and give it one final gentle toss.

Cover the bowl and refrigerate for at least one hour before serving.