Bring 2 cups water to boil in large saucepan. Add cut cauliflower, then sprinkle with salt. Cover and bring to full boil again. Boil 1 minute, drain in colander, then rinse in water until cool. Set aside. Alternatively, you can use an Instant Pot and steam the whole head of cauliflower for 1 minute, then rinse in cool water before cutting.
Whisk mayonnaise and vinegar in large bowl until smooth. Mix in salt and onion powder.
Add cauliflower, celery, and egg. Toss to evenly distribute dressing into cauliflower.
Mix in celery seed, if using.
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Notes
Be careful not to overcook the cauliflower. If you have an Instant Pot, I find it's the best cooking method. Just place 2 cups of water in the pot, place the head of cauliflower on the rack, and cook on "steam" for 1 minute followed by a quick pressure release.Bacon, onion, and pickles are great add-ins. You can also sprinkle on some paprika onto the finished salad as a garnish