To many people, the loin of lamb, with its sweet, delicate flavour, is the best part of the animal. Though there are many recipes and ways you can cook loin of lamb, below is a simple method that will assure you praiseworthy results time and again.
Buy a boneless lamb loin or ask your butcher to remove the bones from your roast when you purchase it. Most meat counters or butchers will be more than glad to perform this service if a boneless lamb loin is unavailable.
Preheat your oven to 162 degrees C (325 F). Place your baking bag in a large baking dish. If your loin is quite large, you might need to place it in a turkey-size baking bag. The baking bag box will tell you how big much meat the bags will accommodate.
Cut several slits in the roast with a knife. Place a clove of garlic inside each slit. (There's no right or wrong number of garlic cloves to use. The more you use, the stronger the garlic flavour will be.)
Sprinkle salt and pepper over the entire surface of the lamb loin and rub it in. Stuff some sprigs of rosemary into the slits with the garlic cloves, if you desire.
Place your lamb loin in the baking bag. Place several springs of rosemary into the bag and on top of the loin. Seal the bag with the provided twist tie.
Make three slits in the top of the baking bag so steam can escape. Lamb loins weighing 1.81 to 2.27 kg (4 to 5 lbs) will take approximately 1 to 1 1/2 hours to cook. Use a meat thermometer to check for doneness: Stick the thermometer into one of the slits in the bag and then into the meat. Once the loin is done, remove it from the oven and allow it to sit in the bag for approximately 15 minutes before you carve and serve it. This will allow the juices to settle and make the loin more tender.