Pour off all but 2 tablespoons of the fat (leaving the beef juices in the pan) from the roasting pan and discard the remaining fat (or reserve if making Yorkshire Pudding).
Place the roasting pan on two (2) burners on medium heat. Add the beef stock and stir and scrape the bottom of the pan to release any browned bits in the pan. Add red wine of your choice.
Bring mixture to a boil and cook until the stock is slightly reduced, about 5 minutes. Au jus is not thick like a typical sauce or gravy.
Season to taste with salt and pepper.
Transfer to a gravy boat or serve in small individual containers for each guest.
Use any left over Prime Rib and Au Jus for making French Dip Sandwiches for your family the next day. So good!
* The beef au jus will only be as good as your beef stock (and also the red wine you use), so it is recommended that you use homemade Beef Stock, if possible. If you must use canned stock, buy the best you can find, but forget about using salty, artificially-flavored bouillon cubes.
** I like to add some of the same red wine that I will be serving with the meal.
Beef Au Jus Recipe: https://whatscookingamerica.net/sauces_condiments/beef-aujus.htm