Gravy is so important to the holiday meal that it gets its own slot on the pot-luck sign-up sheet. Not just anyone can be entrusted to bring that rich and creamy dish (is it a side or a condiment?) that is best served from Grandma's heirloom gravy boat. All the hard work you put into roasting a flavorful Thanksgiving turkey shines through here by using the drippings as the basis of your gravy and building deep, wonderful flavor. This makes plenty for everyone to enjoy and spoon over their mashed potatoes, cornbread dressing, or hot biscuits. Deliciously golden brown mushrooms are cooked into this savory gravy which is sure to be a hit. The touch of sherry vinegar right at the ends helps to brighten and balance this rich gravy. Be sure to look for unsalted chicken stock if using store bought. Because you are using pan drippings, you really want to be in control of how much salt you are adding. Particularly salty pan drippings will require less salt and you don't want to be fighting salt from your drippings and your stock; it is best to taste the pan drippings before adding them to the gravy mixture, that way you know if you need any added salt. Plus, by using the pan drippings you don't have to add any extra fat. When you collect the turkey drippings scrape the bottom of the roasting pan to get any flavorful browned bits. Melt unsalted butter, add sliced mushrooms, and cook until golden brown. Sprinkle in flour and stir until well-incorporated. Gradually add your stock and pan drippings, and simmer until thickened.