Preheat the oven to 200 degrees. Lay the garlic, bay leaves, thyme, and duck fat in the bottom of an enameled cast-iron pot. Sprinkle evenly with the peppercorns and salt. Lay the duck on top, skin side down. Add enough olive oil to cover the duck completely. Cover the pot with foil and bake for 12 to 14 hours, or until the meat pulls away from the bone. Remove the duck from the fat and strain. Reserve the duck fat.