Heat vegetable oil in a large stockpot over medium-high heat. Brown turkey neck, about 6 minutes per side. Add the onion, celery, and carrots; cook until the vegetables begin to soften and brown, 5 to 7 minutes. Pour in the Marsala wine, and scrape and dissolve any browned meat and vegetable bits off the bottom of the pan. Raise heat to high; cook until liquid is reduced by half.
Pour roast turkey drippings into fat separator and let stand until fat separates from turkey drippings
Stir in 2 quarts of cold water, bay leaf, and 1/4 ounce dried porcini mushrooms and optional giblets. Add separated turkey drippings, bring to a simmer and turn heat to low. As stock simmers, skim and discard any foam that comes to the top. Cover partially, and simmer on very low heat for about 1.5 hrs. Strain out solids, and set broth aside to cool.
In a bowl, cover remaining 1/4 ounce dried porcini with 1 cup warm water; allow to rehydrate for 20 minutes. Remove mushrooms from water, and chop fine.
Heat 1/4 cup butter in a large saucepan over medium heat, and brown mushrooms, about 10 minutes. Stir flour into mushrooms and butter, and cook over medium heat, stirring constantly for 3 minutes to make roux. Whisk in broth, about 1/2 cup at a time, and raise heat to medium high; bring gravy to a boil.
Reduce heat to a simmer, and cook gravy until reduced and thickened, about 30 minutes. Stir often. Just before serving, stir in cream, and season with salt and black pepper.