For the stock; put all the ingredients into a medium pot, cover with water, about 10 cups, and bring to a boil. Skim off the scum that floats to the surface, and simmer the stock until the chicken has cooked, about 1 1/2 hours. Leave the chicken to cool in the stock.
For the soup, sweat the diced carrot, celery and onion in the oil in a medium pan together with the garlic until the vegetables are soft. Add the sliced shiitake mushrooms and fry until cooked.
When the stock is ready, strain the liquid over the vegetables and mushrooms. You'll need 8 cups.
Keep the chicken carcass and pick off the cooked meat in small pieces. Add this to the soup with the spinach, when the spinach has wilted in the soup add the nutmeg, salt and pepper.
As an variation you may also add a poached egg or some grated parmesan on top of the soup.