In a small bowl, mix the kosher salt, crushed black peppercorns, smashed garlic, fresh thyme leaves, and crumbled bay leaf. Rub this seasoning mixture evenly all over the duck legs, pressing it gently into the skin and meat. Place the duck legs in a glass dish, cover tightly with plastic wrap, and refrigerate for at least 12 hours or overnight to cure.

Preheat your oven to 300°F (150°C). Remove the duck legs from the refrigerator and rinse them thoroughly under cold running water to remove all of the salt cure. Pat the duck legs completely dry with paper towels to ensure they will crisp up beautifully later.

Place the dried duck legs snugly into an oven-safe baking dish or Dutch oven in a single layer. Pour the melted rendered duck fat over the legs, ensuring they are completely submerged in the fat. If they are not fully covered, you can add a little melted butter or neutral oil to top it off.

Cover the dish tightly with a heavy lid or aluminum foil and place it in the preheated oven. Bake for 1 hour and 45 minutes, or until the duck meat is incredibly tender and easily pulls away from the bone. The fat should be gently simmering throughout the process, not boiling rapidly.

Carefully remove the dish from the oven and use tongs to transfer the duck legs to a wire rack set over a baking sheet. Increase the oven temperature to 400°F (200°C) and roast the duck legs for an additional 15 minutes until the skin is deeply golden brown and crispy. Serve warm with duck-fat roasted potatoes and a sharp, mustard-dressed green salad to cut through the richness.