Step 1Combine the potatoes, cucumber, onion, celery seed, salt and pepper in a large bowl. Toss to combine.
Step 2Separate the yolks and whites of the eggs. Dice the whites and add to the potatoes. Mash the yolks well in a separate bowl and add the sour cream, mayonnaise, vinegar and mustard. Mix well.
Step 3Add the dressing to the potato mixture and mix well. Let stand in the refrigerator at least 1 hour before serving.