Heat vegetable oil in a large stockpot over medium-high heat. Brown turkey necks in hot oil, about 6 minutes per side. Add the onion, celery, and carrots; cook until the vegetables begin to soften and brown, 5 to 10 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.
Stir in 2 quarts of cold water, bay leaf, garlic cloves, and 1/4 ounce dried porcini mushrooms. 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 4-5 hours. Strain out solids, and set broth aside to cool.
In a bowl, cover 1/4 ounce dried porcini with 1 cup hot water; allow to rehydrate for 10 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. 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.
