Preheat the oven to 375 degrees F.
Coat a large ovenproof skillet with the duck fat and place over medium heat. When the fat is nice and hot, add the onions, tossing to coat, and cook for 5 minutes until they begin to caramelize. Add the thyme, and season with salt and pepper. Stick the pan in the oven and roast for 15 minutes until the onions are soft and slightly charred.