1. Season the roast liberally with kosher salt and place it in the refrigerator, uncovered, for at least one hour (and up to overnight). Allow your butter to soften at this time as well.
2. Set the sous vide to 132°F (for medium rare).
3. Seal the roast in a large vacuum bag or two ziploc bags (doubled up), then place it in the water bath.
4. Cook for 6–10 hours in a sous vide machine. If the roast is frozen, it will take 8 to 12 hours to cook.
5. Remove the beef from the bag and pat dry, reserving the cooking liquid.

Finishing:

1. Preheat the oven to 550°F.
2. Combine butter, garlic powder, rosemary, thyme, pepper, and herbs de provence in a small bowl. Place the roast in a roasting pan and brush the top and sides with the garlic herb butter.
3. 8-10 minutes, or until the crust is golden brown, roast the prime rib (keeping a close eye on it).
4. Remove from the oven and set aside to cool.
5. Make the au jus while the roast is resting. Bring the sous vide cooking liquid and broth to a boil in a large skillet, then reduce to a low heat and simmer until reduced by half. Remove any solids that float to the surface.
6. On the side, serve sliced prime rib with au jus and horseradish.