While the prime rib is resting, pre-heat a large skillet over medium heat and add 1/4 cup of beef drippings from the roasting pan you used to make the prime rib. If you don't have beef drippings, use 1/4 cup of unsalted butter.
If using butter, let it melt before proceeding. Vigorously whisk the flour into the beef drippings and cook for 2-3 minutes until golden in color.
Slowly add the beef stock to the flour roux and vigorously whisk. Add the stock slowly as this is where the lumps can happen! Whisk in the mustard.
Add the thyme, rosemary and smashed garlic cloves and bring to a gentle boil, before lowering the heat to a simmer.
Cook and whisk constantly, until it begins to thicken, approx. 3-4 minutes.
Transfer to a gravy boat and serve with the prime rib roast.
Notes
Note 1 - Prime rib does not give off a ton of drippings. You can use 1/4 cup of unsalted butter instead.Note 2 - Use low sodium beef broth for the gravy as the prime rib roast will have a TON of salt. You don't want to overload the dish with even more salt.Note 3 - If you're out of rosemary, check out this list of Substitutes For Rosemary.