Preheat oven to 400. Mix all ingredients for spice mix together and set aside
Open leg of Lamb and coat with 1 1/2 to 2 generous tablespoon of the spice mix.
Place most of the chopped dates on the leg of Lamb along with the chopped rind of the preserved lemon. Roll and tie shut.
Place the Lamb, sweet potatos, and onions in a greased roasting pan
Take 1 teaspoon of the spice mix, about two tablespoons of the juice from the preserved lemon, and a splash of olive oil along with 2 tablespoons of the chopped dates. Mix together and pour over the Lamb and vegetables.
Roast in a 400 oven until meat thermometer reads 135 to 140.