Preheat your oven to 350°F (175°C). Rinse the duck inside and out under cool water, then pat it completely dry. Rub butter over the entire bird, then season generously with salt and pepper, and sprinkle some thyme on top. Stuff the cavity with rosemary, sage, and garlic.
Place the duck neck (if you have it) and the quartered onions on the bottom of your roasting pan to create a natural trivet.
Lay the duck breast-side down on top of the onions.
Roast in the preheated oven for 1½ hours.
Remove the pan from the oven and carefully transfer the duck to a plate. Pour most of the rendered fat into a jar and reserve it for later use (it’s excellent for roasting vegetables!).
Add the sliced apples to the pan, stirring them gently with the onions to coat them in the remaining drippings.
Return the duck to the pan, this time breast-side up, and continue roasting for another 30 minutes, or until the duck is fully cooked and the internal temperature reaches 165°F (74°C), or the juices run clear. For extra crispy skin, you can finish the duck under the broiler for 2–3 minutes, keeping a close eye so it doesn’t burn.
Once the duck is done, remove it from the oven and transfer it to a cutting board. Let it rest for at least 10–15 minutes before carving. Remove some of the caramelized onions and apples from the pan to serve alongside the duck.