Step #1 Steam/boil the cauliflower flowerets for just 3-5 mins (you just want them slightly parboiled -- like you would do to prepare them for freezing).
Step #2 Drain.
Step #3 Rinse w/ cold water; drain & cool.
Step #4 Meanwhile, lemon juice, sugar, in a small bowl, both mustards, whisk together the sour cream, salt & pepper; set aside.
Step #5 In a large mixing bowl, toss together the cooled flowerets, onion & celery.
Step #6 Pour the dressing over veggie mixture & carefully toss to evenly coat w/ dressing.