Bone-in lamb loin is a large cut of meat that is more tender when cooked than traditional lamb loin. Traditional lamb loin is rolled with the bone removed, which can cause the meat to dry out faster. While there are other ways to prepare bone-in lamb loin including smoking and curing, the only practical method for most home chefs because of the size is to bake the meat in the oven. The bone-in lamb loin will require you to cook the lamb loin a little longer than traditional boneless lamb loin, but the extra flavor and succulent meat is worth an extra few minutes per pound.
Remove the bone-in lamb loin from the refrigerator and wrap it with plastic wrap. Set it on counter to allow it to warm up to room temperature, which should take about an hour.
Preheat the oven to 425 degrees Fahrenheit.
Remove the plastic wrap and brush the outside of the bone-in lamb loin with the melted unsalted butter and extra virgin olive oil on a cutting board or clean work surface. Rub the outside of the bone-in lamb loin with sea salt or kosher salt and freshly ground black pepper to taste.
Place the bone-in lamb loin in a roasting rack in a large roasting pan. Tighten the sides of the roasting rack to gently squeeze the lamb loin.
Put the lamb loin in the oven on the center rack for 90 minutes. Do not open the oven door as it is cooking.
Check the internal temperature of the bone-in lamb loin with an instant-read thermometer at the thickest point without touching the bone. The lamb loin should be a minimum of 120 degrees F inside for rare meat, 130 to 135 degrees F for medium rare and 140 to 145 degrees F for medium.
Return the bone-in lamb loin to the oven in 10 minute intervals if the meat is not cooked to your liking. Check the temperature every 10 minutes. Allow the bone-in lamb loin to rest at room temperature for 15 minutes after it is finished cooking.