Bring a large pot of salted water to a boil and add the ditalini pasta. Cook according to package directions until al dente—you want it tender but still with a slight bite, not mushy. Drain the pasta thoroughly and rinse it under cold running water until completely cooled. This stops the cooking process and prevents the pasta from becoming sticky. Set aside.

In a large mixing bowl, combine the mayonnaise, Dijon mustard, cider vinegar, sugar, and salt. Whisk these together until smooth and well blended. I like to add the grated onion directly to the wet ingredients so it dissolves and distributes evenly throughout the dressing rather than remaining gritty. Then stir in the celery seeds and black pepper, adjusting the seasoning to taste if needed.

Add the cooled pasta from Step 1 to the dressing bowl and toss gently but thoroughly to coat every piece evenly. Then add the coleslaw mix and fold it in gently to combine without crushing the vegetables. I find that folding rather than stirring vigorously helps keep the coleslaw texture crisp and intact. Taste and adjust seasoning as needed—the salad should taste bright and slightly tangy.

Transfer the pasta salad to a serving bowl or storage container. For best flavor development, chill for at least 30 minutes before serving—this allows the flavors to meld and the vegetables to soften slightly. The salad can be made a few hours ahead and stored in the refrigerator.