Preheat oven to 220°C. Place lamb in a large baking dish. Use a sharp knife to make 10 x 4cm incisions into lamb.
Juice 1 lemon half. Combine with 1 tablespoon oil, garlic oil, salt and pepper. Cut remaining lemon into 10 small pieces. Push lemon pieces into incisions. Brush lamb with oil mixture and roast for 15 minutes.
In remaining oil, toss potatoes, oregano, salt and pepper. Arrange potatoes around lamb. Return to oven and roast for 40 minutes.
Add carrots to roasting pan with lamb and potatoes. Roast a further 20 minutes. Remove lamb from oven. Increase oven temperature to 230°C.
Cook potatoes and carrots a further 15 minutes or until crisp. Cover lamb loosely with foil and rest for 15 minutes.
Carve lamb, reserving 400g for the Shepherd's Pie (see related recipe). Serve with potatoes and carrots.
Leftovers: Wrap cooled lamb in baking paper, then foil. Refrigerate in an airtight container for up to 3 days.