Bring a large pot of salted water to a boil. Add cauliflower and cook until fork-tender, 5 to 7 minutes. Drain, rinse under cold water, and drain well.

Chop the hard-boiled eggs, red onion, and celery into small, even pieces.

In a large bowl, whisk together mayo, mustard, vinegar, salt, and pepper until smooth.

Add the cauliflower, eggs, onion, celery, and 1 tbsp dill to the dressing. Fold gently until coated. Cover and refrigerate at least 30 minutes.

Scoop into a serving bowl, sprinkle with the remaining dill and paprika, and serve.