1. Coarsely chop the onion. Slice the oranges into thin rounds.
2. In a large saucepan, melt the duck fat. Add the chopped onion and cook over medium heat until the onion is translucent. Remove the onions and set aside in a bowl.
3. Place the duck breasts in the same saucepan, skin side up. Cook for 10 minutes. Season with salt and pepper generously. Cover the saucepan. Cook for 10 minutes over medium heat.
4. Add the cooked onion, orange juice, and Grand Marnier. Bring to a boil over high heat. Reduce the heat, add the orange slices, and simmer for 2 minutes.
5. Remove the duck breasts from the saucepan. Let rest for 4 minutes before slicing into thick slices.
6. To serve, place the sliced duck breasts on a serving plate. Place the orange slices on top of the meat and drizzle with the sauce.