Begin by placing the duck legs or breasts on a cutting board, skin side up. Using a sharp knife, carefully score the skin in a crosshatch pattern, taking care not to cut into the meat.

Generously coat both sides of the duck with salt, ground black pepper, oregano, thyme, and minced garlic. After seasoning, arrange the sliced onion on the bottom of a ziplock bag and place the duck on top. Seal the bag and refrigerate for 24 to 48 hours.

After the marinating period, preheat your oven to 375°F (190°C). Remove the duck from the fridge and gently shake off the excess seasoning and onion.

In an ovenproof baking dish, snugly place the duck pieces skin side up. Pour the duck fat over the duck, which should cover them generously. Roast in the preheated oven for 30 minutes.

Lower the oven temperature to between 275°F and 290°F (135°C to 143°C), cover the baking dish with aluminum foil, and continue roasting the duck for 2 hours or longer, until tender.

Once the meat is tender, carefully remove the foil and increase the oven temperature back to 375°F (190°C). Roast for an additional 20 to 30 minutes until the skin turns golden-brown and crispy.