Sift the flours, almonds, fruit sugar and xanthan gum into the bowl of a food processor and blend well. Add the dairy-free margarine and blend until the mixture resembles fine breadcrumbs. Add the egg and and then 2 tablespoons chilled water and blend for 20–30 seconds until the mixture comes together to form a light, very sticky dough.
Shape the pastry into a ball, wrap in cling film and chill in the fridge for 30 minutes.
Preheat the oven to 180°C/350°F/gas 4 and arrange the nectarines over the base of a 20 x 30cm/8 x 12in tart tin. Sprinkle the sugar and cornflour over, covering the fruit evenly.
Dust a large chopping board with rice flour and gently roll the pastry in the rice flour a little, so that it's just possible to roll out initially. Roll the pastry to about 3mm thick, large enough to cover the tart tin. When the pastry becomes too sticky to work with (which it will!), cover it with a sheet of baking parchment so you can roll over it. Pick up the baking parchment and carefully lift it and place over the fruit in the tart tin (the pastry will stay stuck to the parchment enabling you to do this). Press down firmly round the sides of the tin to cut the pastry and carefully peel the parchment off the pastry, leaving the pastry on top of the fruit. Using a knife, remove any extra pastry over the sides of the tin.
Bake for 30 minutes then remove from oven, wrap in baking parchment, and bake for a further 20 minutes until the pastry is golden brown and the fruit is tender (use a skewer to test this). Serve either hot or cold.
Recipe by Grace Cheetham at https://www.gracecheetham.com/gluten-free-dairy-free-nectarine-tart