1. In a covered saucepan, cook the whole, unpeeled potatoes in boiling salted water for 25 to 30 minutes or until fork tender. Drain and let cool enough to handle.
2. Peel the potatoes with a paring knife and cube them. Transfer to a large bowl. Add the chopped onions and 6 chopped hard-boiled eggs.
3. In a separate bowl, combine the mayonnaise, sugar, celery seed, apple cider vinegar, yellow mustard, salt, and curry powder. Mix well until smooth.
4. Add the mayonnaise mixture to the potato mixture. Toss lightly to coat the potatoes evenly.
5. Cover the salad and chill thoroughly in the refrigerator, preferably overnight.
6. Before serving, garnish with the remaining hard-boiled egg slices. Sprinkle lightly with paprika and parsley, if desired.