Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer; bring water to a boil. Add cauliflower; cover and steam until tender, about 7 minutes. Immediately rinse with cold water; set aside to cool.
Combine celery, bell pepper, and onion in a large bowl; stir in mayonnaise, vinegar, mustard, salt, and black pepper. Lightly toss in cauliflower and eggs until well combined.
Cover the bowl with plastic wrap; chill in the refrigerator until flavors meld, 6 to 24 hours.
Creamy Fauxtato Salad · Dinner Match Lab | Dinner Match Lab