Begin by patting your bone-in prime rib roast dry with paper towels. Generously season all sides with kosher salt, ensuring an even coating.

While the roast brines, combine freshly ground black pepper, garlic powder, onion powder, dried rosemary, and dried thyme in a small bowl. Mix well.

After brining, remove the roast from the refrigerator and let it sit at room temperature for about 3 hours. Preheat your oven to 250°F (120°C). Drizzle olive oil over the roast and rub the spice mixture on the surface.

Confirm that your oven is set to 250°F (120°C). Place the roast on a wire rack over a baking sheet to roast evenly.

Insert a meat thermometer into the thickest part of the roast and roast until the internal temperature reads 120°F for rare or up to 130°F for medium doneness.

Once your roast reaches the desired doneness, remove it and rest for about 30 minutes to allow juices to redistribute.

While the roast rests, heat a skillet over medium-high heat and sear the roast on all sides for 1–2 minutes until a golden-brown crust forms.

In the same skillet, sauté chopped mirepoix until softened, about 5 minutes. Add garlic and deglaze with red wine, scraping flavorful bits.

Pour in beef broth and Worcestershire sauce; bring to a simmer and let it reduce for about 10-15 minutes.

To thicken, mix cornstarch with cold water and whisk into the simmering broth. Cook for an additional 2–3 minutes until it thickens slightly.

In a mixing bowl, combine sour cream, prepared horseradish, lemon juice, and chopped chives. Stir until well mixed and chill.

Carefully carve the rested prime rib against the grain into thick slices. Serve with au jus and horseradish cream sauce.