1 9-lbbeef rib roastabout 4 ribs, meat cut from bone, meat and bone tied together with kitchen twine
1can14 oz low sodium beef broth
2tspwhole pink peppercornscoarsely crushed, plus more for garnish
Place rib roast, fat side up, in a roasting pan and let stand at room temperature 1 hr.
Heat oven to 450ºF and place rack in center. Sprinkle roast with 2 tsp salt. Combine mustard and garlic; evenly spread over top of roast and season with black pepper.
Roast 20 min, then reduce temperature to 325ºF. Roast until an instant read thermometer inserted in center by 2 inches (without touching bone) registers 125ºF to 130ºF for medium-rare (loosely tenting with foil if top browns too quickly), about 2½ hr. Transfer to a cutting board, tent with foil and let stand 15 min before carving (Internal temperature will rise 5 to 10 degrees.) Reserve pan and juices.
Meanwhile, for Creamed Horseradish, whisk cream until thickened but not yet holding soft peaks, 1 to 2 min. Gently fold in horseradish and remaining ½ tsp salt; season with black pepper. Transfer to a serving bowl; chill 30 min or up to 1 hr.
For pan sauce, pour pan juices into a fat separator; drain off fat. Add cognac and ½ cup broth to a 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 min. Add remaining broth, the pan juices and peppercorns; simmer 2 min more. Transfer to a gravy boat.
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.