Add 1″ of water to the bottom of a medium pot (with lid). Insert steamer basket and fill with chopped cauliflower. Cover.
Steam cauliflower on the stove top over medium-high heat until slightly tender, about 10 minutes after water begins to simmer (overcooking will develop a stronger "cauliflower" smell, and a mushy texture).
Drain cauliflower and rinse with cold water to cool immediately. Place in a large bowl
Add celery, onion, parsley, and egg.
Stir in mayonnaise, Dijon mustard, and sea salt (if desired).
Serve immediately or store in refrigerator.