On a wooden board, finely mince the herbs and spices. Rub the mixture into the surface of the duck, then rub the salt vigorously over all surfaces of the duck, placing it in a large (or 2 smaller) ceramic or earthenware casserole. Cover the casserole and hold in a cool place or in the refrigerator, permitting the "marinade" to perfume and soften the flesh for 3 days.
Remove any excess salt still adhering to the duck. Warm the rendered duck fat and the lard in the largest, heaviest pot in the house. Immerse the duck in the warm fat and cook it over the lowest flame for 2 hours or until the flesh is meltingly tender and beginning to fall from its bones.
Lift the duck from its bath and pack it into clean crocks or jars, ladling the warm fat over it, immersing it completely. Cover the crocks or jars with oiled parchment, using butcher's twine to secure the paper. The batu is ready to serve or keep, always in a cool place, for several months.