Cook potatoes in jackets in boiling water until tender. Meanwhile hard cook eggs if not previously done. Drain potatoes, peel, and slice.
Place sliced potatoes in a large bowl, pour dressing over warm potatoes; chill 2 hours.
Peel eggs and separate yolk from whites. Chop egg whites and set yolks aside.
Add celery, onion, and chopped egg whites to potatoes.
In a small bowl, sieve yolks; mix with mayonnaise, sour cream, and horseradish mustard; fold into salad.
Add celery seed to taste.
Chill 2 hours.
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