Start by making your blueberry jam. In a medium saucepan combine your blueberries, ½ cup sugar, lemon juice, and lemon zest. Mash by hand so that all berries are crushed. Simmer on medium low heat for about 30 minutes or until thickened.
Take your jam off the heat and strain jam at least two times! This is very important unless you want to be chewing on unpleasant blueberry skins.
In a separate VERY large sauce pan, add your cream, remaining 2 cups of sugar, blueberry jam and whisk together. Bring it to a boil and whisky heavily and continuously.
Once the mixture is boiling for about 5 minutes remove from heat and add your vanilla.
Let cool for about 20 minutes. Then pour the mixture on to your crust. Let chill in the freezer for about 8 hours or overnight