Add the duck fat to a large, straight-sided pan over medium heat. Add the onions, season with salt, to taste, and saute until the onions are soft, translucent and are very aromatic, about 8 to 10 minutes.
Add the potatoes and toss to coat with the duck fat and combine with the onions. Season with salt, to taste. Shake to spread the potatoes into an even layer and press down gently with a spatula to the bottom of the pan. Let the potatoes brown on the bottom, shaking frequently to prevent sticking. Toss the potatoes several more times, press them down again to allow for maximum browning. Don t worry if the potatoes lose their perfect square shape, that s how they roll. When the potatoes are cooked through, about 12 to 15 minutes, taste for seasoning and adjust, if needed (they will). Remove them from the pan to a serving bowl and sprinkle with chopped chives.