In a large pot, bring salted water to a boil. Cook the cauliflower for about 4–7 minutes until fork-tender but still firm. Drain and cool to room temperature.

In a mixing bowl, combine cooled cauliflower, apple cider vinegar, Dijon mustard, mayonnaise, salt, and black pepper. Mix gently until coated.

Fold in red onions, hard-boiled eggs, and celery, adding dill if using. Mix carefully to maintain the cauliflower's structure.

Cover with plastic wrap and refrigerate for at least 2–3 hours to allow flavors to meld.

Before serving, stir gently and garnish with extra chopped dill if desired.