Generously season the roast with kosher salt and place in the refrigerator, uncovered, for at least one hour (and up to overnight). This is a good time to let your butter soften as well.
Set sous vide to 132-degrees (for medium rare).
Place the roast in a large vacuum bag or two ziploc bags (doubled up), seal, and place in the water bath.
Sous vide for 6 - 10 hours. (8 - 12 hours if the roast is frozen.)
Remove the beef from the bag, reserving the cooking liquid, and pat dry.
Finishing:
Preheat oven to 550 degrees.
In a small bowl, combine butter with garlic powder, rosemary, thyme, pepper and herbs de provence. Coat the top and sides of the roast with the garlic herb butter and place in a roasting pan.
Roast the prime rib for 8-10 minutes, until the crust is golden (keeping a close eye on it).
Remove from the oven and let rest.
While the roast rests, make the au jus. In a large skillet, bring the sous vide cooking liquid and broth to a boil, reduce the heat and simmer until reduced by half. Skim off any solids that rise to the top.
Serve sliced prime rib with au jus and horseradish on the side.