In a deep pot, simmer the ham (if using) and half of the onion in a teaspoon of butter until onion is golden along edges but not brown. Add minced garlic and sauté 1 minute. Add water or broth, celery, carrot, thyme, pepper, bay leaf and nutmeg.
Simmer over low heat (do not boil), covered, until the chicken is tender (about 20 minutes or so). How long this will take depends upon the age of the chicken. Remove chicken from pot using a slotted spoon when cooked and set aside to cool just enough so that the meat can be separated from the bones. Remove bones from chicken and return to the pot.
After the chicken is removed from the pot, turn to medium high heat and boil off the liquid until it is reduced in volume by half. If you want to have more vegetables in the pot pie, they can be added to the broth at this time; small diced potatoes, carrot chunks, celery chunks, peas, small white onions (halved), etc., all make good additions (and budget stretchers) in a pot pie.
Meanwhile, combine the butter and flour, rubbing together until smooth. Melt this into the simmering chicken broth to thicken it. Rub the sage leaf to a fine powder and add to the broth. Allow broth to thicken, then remove from heat. Cool before adding to pie.
Prepare the pie crust (below). Line a pie pan with rolled out crust. Fill with cooked, cooled chicken and vegetables. Add cleaned mushrooms, thinly sliced. Add a small portion of the thickened broth over all (not enough to make a soup, but just a gravy). Sprinkle lightly with onion powder, salt and pepper.
Top with the second pie crust; crimp and seal edges, flute if desired. Decorate top with small bits of extra pie crust cut with small cookie cutters, if desired. Cut a vent in the center to allow steam to escape. Brush the pie top with cream or egg wash if an extra sheen is wanted. Bake at 375°F until pie is golden brown and gravy begins to bubbly up from center vent (about 40 minutes). Shield the edges of the pie with aluminum foil if it begins to brown too quickly.
until crumbly. Mix in:
