I recreated this after serving it up one summer from the counter at William Nicholas in Katonah, NY. I make this only in warm weather.
METHOD
Preheat two sheetpans and oven to 400F.
Scrub and dice 1# small red new potatoes. You want a smallish dice - about 1.5cm squares As you cut them transfer to a bowl of cold water.
Rinse until water runs clear over potatoes and then drain. Dry as much as possibly on a kitchen towel. Transfer to a bowl and toss with 1-2 T canola oil.
Remove one sheetpan from the oven at a time. Spray lightly with canola spray and then add half the potatoes onto it, spreading evenly. Transfer full sheetpan to the oven, removing the other.
Bake the potatoes for about 25 minutes, rotating pans as needed and tossing the potatoes several times throughout roasting time.
Potatoes are done when they are nicely browned all over and tender. When fully cooked, remove from the oven and allow to cool to room temperature. This will take a good 15-20 minutes.
The Dressing
Combine in a small mixing bowl:
3-4 T sour cream (I use a light sour cream)
3-4 T mayonaise (Hellmans/Best Foods, always)
2 green onions, finely chopped
Kosher salt and fresh ground pepper
When the potatoes are cooled, transfer them to a serving bowl. Add about half of the dressing and stir to coat. Add as much of the dressing as suits your taste. Add more salt or pepper to taste and serve.