Bring a large pot of salted water to a boil over high heat. Add the cauliflower florets and cook until the cauliflower is soft and releases easily when pierced with a knife, about 8 minutes. Drain in a colander and spread out on a baking sheet. Pat the cauliflower dry with a clean kitchen towel or a paper towel, then set aside and let cool to room temperature, about 30 minutes.
Meanwhile, whisk together the sour cream, horseradish, mayonnaise, chives, cider vinegar, celery, 1 1/2 teaspoons salt and 1 teaspoon pepper in a medium bowl.
Fold the cauliflower into the dressing and season with more salt and pepper to taste. Serve immediately.
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