Bring stock to a boil in a medium pot; continue to boil until reduced slightly, about 10 minutes
Meanwhile, cook flour and fat in a large skillet over medium heat, whisking constantly, until roux has a coppery brown hue, about 5 minutes
Whisk in wine and bring to a boil; cook, whisking constantly, until mixture no longer smells like alcohol, about 2 minutes
Gradually ladle in hot stock, whisking after each addition
Continue to boil, whisking occasionally, until gravy coats the back of a spoon, 20–25 minutes (if gravy gets too thick, whisk in a bit of water or pan juices until it reaches the desired consistency)
Season to taste with salt and pepper
Gravy can be made 3 days ahead; transfer to an airtight container and chill
Heat in a small pot over medium, whisking constantly and adding warm water or pan juices 1 Tbsp
