Heat oil. Add the spices cardamom, cinnamon, cloves and bay leaf. Once the aromas are released switch off heat and cool for 30 minutes. Strain and reserve the oil.
Place the roasted spices, ginger, garlic and yogurt in a mixer and grind to a coarse paste. Add the reserved oil.
Clean the legs, removing excess fat and skin. Make slashes all over to let the marinade soak in. Rub the marinade all over and keep in the fridge overnight or for up to 24 hours. If you want to make it earlier, marinate for a minimum of 8 hours.
Place in a shallow baking tray with enough room to turn while roasting and roast in a hot oven for 15 minutes on each side or until brown all over.
Add a cup of water and continue roasting for another 2 hours, turning the legs every 30 minutes and adding another cup of water as it dries up.
You can add baby potatoes or larger parboiled peeled potatoes cut into halves or quarters during the last 45 minutes.
When roast is done, remove from the pan and deglaze with 2 cups water, scraping all the juices and bits of meat from the bottom of the pan.
Strain the drippings and discard the solids. Add salt to taste and bring to a boil.
Mix 1 tablespoon of corn flour/ cornstarch in 50 ml of cold water and add to the boiling gravy. Once the gravy is thick, turn off heat.
Slice/Carve one leg and discard the bone. Place the other full leg of lamb in a platter and surround with the sliced/carved meat and roast potatoes.