Pat the prime rib roast thoroughly with paper towels until dry. Season the entire surface—including ends, sides, and the underside where the ribs attach—with kosher salt, using only as much as needed. Position the roast in a roasting pan or a 9x13-inch metal baking dish, rib side down and fat cap facing upward. Allow the meat to rest uncovered at room temperature for approximately 2 hours.

In a mixing bowl, combine heavy cream, sour cream, prepared horseradish, ground black pepper, kosher salt, and cayenne pepper if using. Whisk the mixture until it reaches a smooth consistency. Cover the bowl and refrigerate the sauce until serving time.

Place an oven rack in the center position and preheat the oven to 500°F. Insert the roast into the hot oven and cook for 15 minutes. Expect the release of fat and some smoke during this period; ensure proper ventilation by activating the kitchen vent and opening windows.

Reduce the oven temperature to 325°F and proceed to roast for an additional hour before checking the internal temperature. Continue roasting until the thickest part of the roast registers 115°F for rare or between 120°F and 130°F for medium. Note that the meat will continue to cook slightly as it rests.

Once the desired temperature is achieved, remove the roast from the oven and transfer it to a clean cutting board. Tent loosely with aluminum foil and allow it to rest undisturbed for 30 minutes.

Remove any twine and, if preferred, separate the meat from the rib bones in one piece. Slice the roast crosswise into 1/2-inch-thick portions. Serve each portion generously with the chilled horseradish cream sauce or gravy, as desired.