The day before you plan to cook the prime rib, season the roast liberally all over with kosher salt. Place it on a plate or small baking dish and leave it uncovered in the fridge for at least 16 hours. This allows the outside to dry for a better crust and lets the salt permeate past the outer later.

Preheat an oven to 225 degrees F. Place the roast, fat cap up, in a roasting pan with a rack or on a rimmed baking sheet with a wire rack. Nestle the rosemary sprigs under the roast. Mix together the dijon mustard, horseradish, and olive oil and liberally brush all over the roast. Coat with lots of freshly cracked black pepper.

Insert a probe thermometer into the middle of the prime rib. Place the roasting pan in the oven and cook until the internal temperature reaches 120 degrees F (for medium-rare, see chart above for suggested temperatures), about 3 1/2 – 4 hours. Remove the roast from the oven and tent loosely with foil, and allow the meat to rest for at least 30 minutes, up to an hour and a half.

Right before you plan to serve, set the oven to 550 F (or as high as your oven will go) and return the roast to the oven for 10 minutes or so, until the outside is well-browned and crisp. Remove from oven, rest 10 minutes, then carve.

To make the horseradish sauce: whisk together all the ingredients until smooth. Season to taste with salt and pepper. Store in the fridge until ready to serve.