Bring a large pot of salted water to a boil, then add the gemelli pasta. Cook according to package directions until al dente, usually about 8-10 minutes. Once cooked, drain the pasta in a colander and rinse under cold water until fully cooled. Set aside in a large mixing bowl.

In a separate mixing bowl, combine 1 cup of mayonnaise, 2 tablespoons of Dijon mustard, 1 teaspoon of apple cider vinegar, and 1 tablespoon of sugar. Whisk together until creamy and smooth, adjusting flavors if necessary.

Fold the cooled pasta into the dressing, ensuring every piece is well-coated. Add 4 chopped hard-boiled eggs, 1 cup of diced celery, ½ cup of chopped fennel, and ¼ cup of finely chopped red onion. Gently stir to combine.

Sprinkle with kosher salt and freshly ground black pepper to taste, and stir once more to evenly distribute.

Cover with plastic wrap and refrigerate for at least 30 minutes to allow the flavors to meld.

Once chilled, remove from the refrigerator and give it a gentle stir. Garnish with sliced scallions and paprika before serving.