1.Preheat oven to 180°C (160°C fan-forced).
2.Heat oil in a medium frying pan over medium heat; cook onion and garlic, stirring, 5 minutes or until onion softens. Add spinach; cook, stirring, until wilted. Remove from heat; cool 20 minutes. Season to taste.
3.Combine extra garlic, salt, paprika, cumin seeds, thyme and oil in a small bowl.
4.Place lamb, cut-side up, on a board; place spinach mixture down centre of lamb; roll tightly, from short side, secure at 2cm intervals with kitchen string. Rub spice mixture all over lamb. Place lamb on an oiled wire rack in a large baking dish; add the water to dish. Roast for 1¾ hours or until cooked as desired. Cover; stand 10 minutes.
5.Meanwhile, boil, steam or microwave potato and kumara until tender; drain. Using potato masher, mash potato and kumara together roughly with butter. Season to taste.
6.Serve sliced lamb with mashed vegetables; drizzle with pan juices.