Combine the flour, Natvia and butter in a food processor and process until the mixture resembles bread crumbs. Add the egg and vanilla and pulse until the dough starts to come together.
Add 1 tablespoon of iced water if the mix is too dry, or a sprinkling of flour if it is too sticky. Roll into a ball and then flatten into a disk. Wrap in plastic and then refrigerate for 45 minutes -1 hour. Preheat the oven to 180C.
Roll out into a disc until it can fit over a 20cm tart tin. Lay over the tart tin, leaving the ends hanging over the top. Prick the middle with a fork and then place baking paper on top of the tart base. Fill with baking weights or dried beans. Place in the oven for about 15 minutes, remove the baking weights and bake for a further 5 minutes. Remove from the oven.
Whisk the eggs together with the Natvia in a saucepan until thick. Add the lemon zest and some of the milk and place the saucepan over low heat, stirring until combined. Continue to add the milk gradually, constantly whisking. Continue to whisk over low heat until the custard has thickened.
Pour the custard into the tart tin and sprinkle with the dried pine nuts. Place in the oven and bake for a further 25-30 minutes. Remove form the oven and allow to cool completely.