1- In a large saucepan, place yogurt, flour, tomato paste and water. Whisk thoroughly.
2- Add peeled onions, whole. Place on heat and bring to a boil. Remove from heat.
3- Place leg of lamb into saucepan. Place on hot-plate until cooked.
4- Remove lamb from saucepan and place on baking dish. Pour some of the liquid from saucepan over lamb. Place in medium hot oven until lamb browns. Remove from oven, and allow to rest for 5 minutes. Carve and serve with remaining sauce.