Remove pie crust from the freezer and thaw for 15 minutes. Prick thawed shell on the bottom and sides with a fork.
Meanwhile, cut potatoes into 2 inch chunks and steam over boiling water, about 20 minutes.
Place sweet potatoes, milk and cream into your Vitamix container and secure lid. Select Variable 1. Turn machine on and quickly increase speed to Variable 10, then to High. Blend for 1 minute so that potatoes are nicely pureed. (If you do not have a Vitamix, you may pureé in your food processor.)
Whisk the eggs and sugar together in a large mixing bowl. Add 1 1/2 cups of sweet potato mixture, butter, lemon juice, vanilla, cinnamon, nutmeg and salt. Stir and pour into the pie shell.
Bake for 20 minutes.
Reduce oven temperature to 325℉ and continue baking for another 20 minutes.
Let pie cool on a rack and serve with whipped cream.