• Using a sharp knife, cut slits, spaced 1-inch apart, in the surface of fat cap in a crosshatch pattern, being careful to cut down to, but not into, the meat.
• Rub the coarse salt over the entire roast and into the slits. Transfer to a large plate, and refrigerate, uncovered, for at least 24 hours or up to 4 days.
• When ready to roast the prime rib, remove the meat from the refrigerator and let stand on the kitchen counter for 1 hour. 
• Adjust the oven rack to the lower position and heat the oven to 220°F. Coat a rimmed sheet pan with non-stick oil. 

Prepare the Horseradish Crust Mixture:

• Place the softened butter in a bowl, add the mustard, both of the horseradishes, and the anchovies.  Mash the ingredients together until well combined then stir in the thyme leaves. 

Coating the Prime Rib:

• Use paper towels to pat the surface of the roast to remove as much moisture as possible.  This is key for getting the mixture to spread evenly over the prime rib.
• With a silicone spatula or with your hands, slather the horseradish mixture over the top and sides of the roast.
• Place the roast on the sheet pan, bones down.  Insert a thermometer meat probe into the top of the roast to the center of the meat without touching fat or bone.  
• Transfer the roast to the oven. Set the thermometer alarm to sound at your desired doneness. Since this roast gets cooked at low heat, there will not be a lot of carry-over cooking.
• Pull the roast from the oven and let it rest on the sheet pan, tented with foil for 20-30 minutes.

Sear and Crisp the Crust:

• Set the oven temperature to the 'Broil' unit and let heat for 3-5 minutes.
• Remove the foil from the roast and transfer the sheet pan/roast back into the oven and broil, turning from side to side as it colors, until the crust is crisp and a deep brown. Watch closely that it does not burn.
• Remove from the oven, carve and serve. There is no need for a further 'rest' time.