A 15 pound rib roast will easily serve 18 to 20 people, making it a festive choice for holidays and special occasions. A roast this size will likely include almost all 13 ribs, but when given the choice, opt for the loin end of the roast, usually labeled "first cut" or "loin cut." The loin portion is leaner and more tender than the shoulder cut. Start with a very clean oven, because the rib roast produces a lot of smoke as it cooks. Serve a rib roast au jus, with horseradish sauce or Yorkshire pudding.
Remove the roast from its wrappings and trim away most of the fat. If the roast has been dry-aged, you can leave the fat on.
Salt the roast generously and add any additional seasonings, such as rosemary or garlic. Lightly cover the roast and return it to the refrigerator. Refrigerate for several hours or overnight at 40 degrees Fahrenheit. Salting the roast ahead of time allows the flavors to permeate the roast and also makes it more juicy.
Remove the roast from the refrigerator and set it on the counter top for up to two hours before roasting it. Warming the meat slightly before roasting it ensures even browning and cooking.
Preheat the oven to 450 F. Place the roast in a roasting pan, not much larger than the roast. Do not use a rack. Place the roast in the oven and cook it for 20 minutes.
Reduce the heat to 300 F and set the timer for 20 minutes. At this point, insert an instant-read thermometer in the roast, making sure the thermometer doesn't touch bone. Continue checking the roast every 20 minutes until it has reached the desired degree of doneness. Although the USDA recommends cooking beef to 145 F to reduce the risk of foodborne illness associated with eating raw meat, chefs follow the following guidelines: Roast to 120 F for rare; 125 F for medium-rare or 130 F for medium.
Remove the roast from the oven and transfer it to a large carving board. Place the board in a warm, draft-free area and let it rest for at least 20 minutes and up to 40 minutes. .While the roast is resting, use the drippings to make an au jus sauce or Yorkshire pudding.