Place the potatoes into a large pot and cover with salted water. Bring to a boil over high heat, then reduce heat to medium-low, cover, and simmer until tender, about 20 minutes.
Drain and allow to steam dry for a minute or two. Allow the potatoes to cool, then cut into chunks.
While the potatoes are cooking, place the eggs into a saucepan in a single layer and fill with water to cover the eggs by 1 inch. Cover the saucepan and bring the water to a boil over high heat. Once the water is boiling, remove from the heat and let the eggs stand in the hot water for 15 minutes.
Pour out the hot water, then cool the eggs under cold running water in the sink. Peel and chop once cold.
In a large salad bowl, whisk together the mayonnaise and sour cream until smooth, then stir in the apple chunks, red onion, pickle relish, celery, green onions, curry powder, and salt and pepper. Lightly stir in the potato chunks and eggs, toss to coat with dressing, cover the bowl, and chill at least 3 hours before serving.
