Bring a large pot of water to a boil, add the tablespoon of kosher salt, and cook the elbow macaroni according to package directions until very soft and slightly past al dente.

Drain the macaroni in a colander and rinse briefly with cool water just until warm, then let it drain very well.

In a medium bowl, whisk together the mayonnaise, whole milk, apple cider vinegar, granulated sugar, Dijon mustard, black pepper, and the 1/2 teaspoon kosher salt until smooth.

In a large mixing bowl, combine the warm drained macaroni, shredded carrot, diced celery, minced onion, and sweet pickle relish.

Pour the dressing over the macaroni mixture and stir with a wooden spoon until all the pasta is thoroughly coated.

Add the drained tuna, breaking it up gently with a fork if needed, and fold it into the salad until evenly distributed but still slightly chunky.

Stir in the green onions and parsley, then taste and adjust seasoning with additional salt and pepper if desired.

Cover the bowl tightly with plastic wrap and refrigerate for at least 1 hour to allow the flavors to meld and the pasta to absorb some of the dressing.

Just before serving, stir the salad and, if it looks dry, add a splash of milk and a spoonful of mayonnaise to loosen the texture.

Serve chilled or cool, garnished with extra green onions or parsley if desired.