Roasts that weigh around 15 pounds need a balance of lean meat to marbling that works with both high and low heat. Strip loins have a ratio of lean to marbling that suits oven-searing temperatures of around 500 degrees Fahrenheit, and slow-roasting temperatures of around 225 F. They have enough intramuscular fat to prevent drying out when cooked for several hours, and enough leanness towards the surface to produce a crust when seared. Unlike small roasts, you have to cook 15-pound New York strip loins slowly to retain moisture, and increase the heat at the end to sear it.
Heat the oven to 225 degrees Fahrenheit. Pat the strip loin dry with paper towels.
Coat the strip loin liberally with oil. If you have a bone-in strip loin, coat the bones also. Oiled bones create an appealing smoky aroma when roasted.
Season the strip loin to taste. Season the fat cap heavily. As the fat melts, it drips and self-bastes the rest of the strip loin, taking the seasonings with it.
Position one large wire baking rack or two small baking racks on a large lipped baking sheet. The racks keep the strip loin elevated so the heat reaches underneath it.
Insert an oven-safe meat thermometer in the center of the strip loin. If you have a bone-in strip loin, don't let the thermometer probe touch a bone. Roasting with an oven-safe thermometer in situ makes checking the temperature easier.
Place the roast on the wire rack and put the baking pan on the middle oven rack. Roast the strip loin for about seven hours and check the internal temperature. It should measure about 120 F, or rare. Cook the strip loin for about 40 more minutes for medium-rare, or when it measures at 125 F. Cook the strip loin for one hour longer for medium, or 130 F. Cook the strip for one hour and 20 minutes longer for medium-well, or 150 F.
Remove the roast and let it rest for at least 45 minutes. The larger the roast, the longer you have to rest it. During resting, the strip loin will increase in temperature about 10 degrees, and will distribute its juices evenly throughout the meat.
Heat the oven to 500 F while the strip loin rests. It takes about 30 minutes for an oven to go from 200 to 500 F.
Return the strip loin to the oven after it rests for 45 minutes and the oven reaches 500 F.
Oven-sear the strip loin until it develops a golden-brown crust, about 10 to 15 minutes, and remove it. You don't have to rest the strip loin a second time, and can serve it as-is after removing the oven-safe meat thermometer.
The U.S. Department of Agriculture recommends cooking beef to an internal temperature of 145 F.