Prepare pie crust & set aside. Pre heat oven to 190ºC . Wash & scrub potatoes. Place in a baking pan, brush skins with some olive oil & bake for 1.5 hours till very soft. Cool & then peel. Mash & add the rest of the ingredients. Blend till the mixture has a creamy consistency. Reduce oven to 180ºC . Spoon mixture into pie crust. Garnish with walnuts. Bake for 1 hour. Chill or cool to room temperature before serving. Variation. Add 1 to 2 cups cooked chick peas to the sweet potato before cooking the pie. Ensure they rae well mashed.