Preheat oven to 400°.
Pat dry the lamb and place in a sturdy roasting pan. With a sharp knife, make numerous slits all over the lamb and insert garlic slivers. Pour olive oil over lamb to coat. Sprinkle Greek Lamb Rub, ensuring all surfaces are covered.
Add chicken stock to the bottom of the pan, cover tightly with aluminum foil, and place in the oven. Cook for 30 minutes. Reduce temperature to 350°. Cook for another 1 hour and 30 minutes.
While lamb is cooking, peel and cut potatoes in half lengthwise and then in quarters. Place potatoes in a large bowl and toss with salt, black pepper, oregano, and lemon juice.
Remove roast from oven. Increase oven temperature to 400°. Remove foil and nestle potatoes around roast. Add chicken stock or water to pan if liquid has evaporated. Return the pan, uncovered, to oven and cook for 25–30 minutes or until lamb is well browned and crisp.
Remove roast from pan, cover with aluminum foil, and let rest. Place the pan with potatoes back in oven and roast for another 15 minutes or until they are soft on the inside and nicely browned.
Remove potatoes from oven, slice the lamb, and serve with plenty of the juice from the pan.