Season the rib roast with the salt and pepper and let it sit uncovered on a baking rack (over a plate) in the fridge overnight.
Heat a sous vide water bath to 131F degrees.
Place the rib roast in a vacuum seal bag with the roasted garlic and fresh thyme and seal.
Cook in the water bath for 8-12 hours.
Remove from the water bath and place in an ice bath for 5-10 minutes.
Preheat an oven to 500F degrees.
Remove the rib roast from the bag, reserving the bag juice for the jus (below).
Place on a rack over a baking sheet and rub the butter over the top. Roast for 15-20 minutes, until golden brown on the outside.
While the beef roasts, combine all the ingredients for the au jus in a saucepan and bring to a simmer.
Simmer for 5-10 minutes, until reduced. Adjust for salt if necessary (I don't typically add extra salt). Remove from heat.
Remove the rib roast from the oven. Let it cool slightly.
Slice and serve drizzled with the au jus and serve with horseradish sauce (optional).
Salting it overnight allows the salt to penetrate into the meat which adds flavor and also helps keep it tender. While you can skip this step, I recommend doing it if you can so you get the best results possible.
The longer you cook it, the softer it will get. I do not recommend cooking more than 12 hours or it can start to become too soft and mushy.
I recommend cooking it rare to medium rare, but doneness options are included above if you prefer something different.
Always place it in an ice bath when it's done to cool the outside of the pme rib. This allows the butter to be spread on before roasting, as well as helps to avoid it overcooking in the hot oven.
You can cook this ahead and store it in the fridge for up to 5 days until ready to serve it. Reheat it in a 131F degree water bath for 30 minutes, then roast per the recipe instructions.