Remove the silver skin and trim the rib roast. Remove the bones, keeping as much meat on the roast as possible; set the bones aside to cook alongside the roast.
Tie the roast with 3 to 4 pieces of butcher's twine. Don't tie too tight, since the roast will expand while cooking.
Season the roast and ribs with a coarse rub, such as Montreal steak seasoning. Leave uncovered on a rack in the fridge overnight to dry brine (or at least 1 hour).
Combine the horseradish paste ingredients and mix well. Coat all sides of the roast in a thick layer of the paste, leaving the ribs uncoated.
Whisk together the creamy horseradish sauce ingredients and refrigerate until ready to serve.
Place the roast and ribs in a 250°F oven or smoker, with the roast in the middle of the oven. Cook until the roast's internal temperature reaches 123°F for rare/medium-rare or 133°F for medium. Leave the ribs in the oven if they need more time to become tender.
Let the roast rest loosely covered at room temperature for 30 minutes.
Increase the oven to 550°F (or its highest setting) and return just the roast to sear for 5 to 10 minutes, removing it as soon as the color is right.
Slice and serve immediately with the creamy horseradish sauce on the side.