Rub the duck legs with salt, crushed garlic, thyme, bay leaves, and black peppercorns. Cover and refrigerate for 24 to 36 hours.
Preheat the oven to 100°C (210°F). Rinse the duck legs to remove excess salt and pat them dry.
Place the duck legs in a deep ovenproof dish and cover completely with melted duck fat.
Cook in the oven for 2 to 3 hours until the meat is tender and easily pulls away from the bone.
Let the duck legs cool in the fat. For storage, keep them submerged in the fat in the refrigerator.
To serve, remove the duck legs from the fat and crisp the skin in a hot skillet or under a broiler until golden brown.