Season the duck portions with the five-spice powder. Place them skin-side down in a very hot frying pan or casserole to brown the skin. Turn the pieces over. Add the lemon grass, garlic, shallots, mushrooms and ginger to the pan, then cover the duck with the stock. Cover the pan with a lid and simmer very gently for 1½ hours.
Remove the duck from the pan and add the medlar berries or Chinese red dates, black fungus and fish sauce. Season with salt and pepper to taste. Mix the cornflour to a smooth paste with a little water and add to the pan. Bring the sauce to the boil, stirring constantly, and cook until thickened. Return the duck to the pan and simmer gently for 30 minutes.
Add the spring onions to the sauce and garnish the duck with the coriander.