Prepare the duck legs: Rinse the duck legs under cold running water and thoroughly pat them dry with paper towels. This is crucial for even salting and helps the skin crisp up later.

Cure the duck legs: In a small bowl, combine the kosher salt, crushed black peppercorns, thyme sprigs, bay leaves, and smashed garlic cloves. Place the duck legs in a non-reactive dish (glass or ceramic). Evenly rub the salt and herb mixture all over the duck legs, ensuring they are well-coated. Cover the dish tightly with plastic wrap.

Refrigerate the duck legs for at least 12 hours, and preferably 24 hours. This curing process seasons the meat deeply and helps draw out moisture, which is essential for successful confit.

Prepare for cooking: After curing, remove the duck legs from the refrigerator. Gently scrape off any excess salt and herb mixture from the duck legs. Rinse the legs briefly under cold water to remove any remaining salt crystals, then pat them completely dry with paper towels. Moisture is the enemy of crispy skin, so ensure they are very dry.

Preheat the oven: Preheat your oven to 150°C (300°F).

Melt the duck fat: In an oven-safe pot or Dutch oven large enough to hold the duck legs in a single layer, melt the duck fat over low heat. If using a combination of duck fat and oil, ensure the fat is mostly duck fat for the best flavor. The goal is to have enough fat to completely submerge the duck legs.

Arrange the duck legs: Once the fat is melted and liquid, carefully place the cured and dried duck legs into the pot, skin-side up if possible, ensuring they are fully submerged in the fat. Add any remaining herbs or garlic if desired, though they are often removed after curing.

Slow cook the duck: Cover the pot tightly with a lid or heavy-duty aluminum foil. Transfer the pot to the preheated oven. Cook for 2.5 to 3.5 hours, or until the duck meat is extremely tender and easily pulls away from the bone when prodded with a fork. The internal temperature should reach at least 85°C (185°F).

Cool and store (optional, for make-ahead): Once tender, you can let the duck legs cool slightly in the fat, then carefully transfer them to a clean container, pour the strained duck fat over them to cover completely, and refrigerate for up to several weeks. For best results, use within 2 weeks. Ensure the duck is fully submerged in fat to prevent spoilage.

Crisp the duck legs: When ready to serve, remove the duck legs from the chilled fat. Scrape off any solidified fat. You can gently warm the duck fat in the pot or a skillet over medium-low heat, or use a separate skillet. Place the duck legs skin-side down in the warm skillet with a little of the reserved duck fat. Alternatively, place them skin-side up on a baking sheet and broil or bake at a high temperature.

Pan-sear for crispiness: Cook the duck legs skin-side down in the skillet over medium-high heat for about 5-8 minutes, or until the skin is deeply golden brown and very crispy. Spoon some of the hot fat over the flesh side occasionally to help cook it through. Flip carefully and sear the flesh side for 1-2 minutes just to warm through.

Broil for crispiness (alternative method): Preheat your broiler to high. Place the duck legs skin-side up on a baking sheet. Broil for 5-10 minutes, watching very closely, until the skin is golden brown and crisp. Rotate the pan as needed for even crisping. This method is faster but requires constant attention to prevent burning.

Serve: Remove the duck legs from the skillet or oven. Let them rest for a minute or two. Serve hot, typically with a side of potatoes (such as pommes sarladaises, which are cooked in duck fat) or a fresh green salad.