1. Place rib roast, fat side up, in a roasting pan and let stand at room temperature 1 hour.
2. Heat oven to 450 degrees F and place rack in center. Sprinkle roast with 2 teaspoons salt. Combine mustard and garlic; evenly spread over top of roast and season with black pepper.
3. Roast 20 minutes, then reduce temperature to 325 degrees F. Roast until an instant-read thermometer inserted in center by 2 inches (without touching bone) registers 125 degrees to 130 degrees F for medium-rare (loosely tenting with foil if top browns too quickly), about 2-1/2 hours. Transfer to a cutting board, tent with foil and let stand 15 minutes before carving. (Internal temperature will rise 5 to 10 degrees.) Reserve pan and juices.
4. Meanwhile, for Creamed Horseradish, whisk cream in a medium bowl until thickened but not yet holding soft peaks, 1 to 2 minutes. Gently fold in horseradish and remaining 1/2 teaspoon salt; season with black pepper. Transfer to a serving bowl; chill 30 minutes or up to 1 hour.
5. For pan sauce, pour pan juices into a fat separator; drain off fat. Add cognac and 1/2 cup broth to roasting pan. Bring to a simmer over medium-high heat, scraping up any browned bits with a spoon. Simmer until liquid has almost evaporated, about 8 minutes. Add remaining broth, the pan juices and peppercorns; simmer 2 minutes more. Transfer to a gravy boat.
6. Cut twine from roast, remove bone and transfer meat to a cutting board; garnish with peppercorns, if desired. Serve sauce and Creamed Horseradish on the side.