Preheat oven to 150 °C. Rub lamb with olive oil, salt and pepper. Using a sharp knife, make small slits into the lamb.
Push rosemary and garlic into the slits. Place onion on the bottom of a roasting pan with any remaining rosemary and garlic on top.
Pour 500 ml of the wine over and place lamb on top. Cover with foil and roast lamb 2 hours. Remove foil and roast for another hour to allow the lamb to brown.
Cook the Spekko Rice according to the instructions on the pack. For every 1 cup uncooked rice, you will get up to 4 cups of cooked rice.
Remove lamb from roasting pan, cover with foil and allow to rest before carving.
Meanwhile, pour pan juices, with onions, garlic and rosemary into a measuring jug. Allow to cool for a few minutes scoop the fat off.
Pour pan juices into a saucepan and heat for a minute or two. Mix cake flour with a little water to form a paste and slowly add to the pan juices, while stirring.
Add remaining wine and simmer until the sauce thickens. Season to taste.
Serve lamb and sauce with Spekko Long Grain Parboiled Rice.