Place the Mix in a bowl, cut the fats into small pieces using a knife and add them to the Mix. (It is important to keep the fats as cold and hard as possible so leave them in the fridge right up until you need to use them).
Using your fingertips, rub the fats into the Mix until the mixture resembles coarse breadcrumbs. (It is important not to over-rub the fats – any larger pieces left will be incorporated later when the pastry dough is kneaded).
Lightly stir in the egg and then gradually add sufficient cold water to produce a soft but not sticky dough (you will probably need 2-3tbsp in total). Using your hands, move the mixture around the bowl allowing it to form a dough without actually forcing it together. When the dough starts to clean the bottom of the bowl, sufficient water has been added.
The dough will still be quite crumbly, so this kneading stage is crucial. (This may seem very strange, but gluten-free pastry differs to traditional pastry as it actually benefits from being handled). Lightly dust a surface with Mix and knead the dough 1-2 minutes. The pastry will change in texture, becoming much smoother and more pliable.
The pastry is now ready to use. To roll out, lightly dust the surface and the rolling pin with Mix and roll out using smooth short strokes. Keep the pastry moving around the surface but do not turn it over. As the piece gets bigger, use your rolling pin, rather than your hands, to pick it up.
Line 4 tart tins (4" diameter) with the pastry, gently pressing the dough around the sides of each tin.
Brush the edges with the remaining egg and bake in a preheated oven (200°C/180°Fan/Gas Mark 6) for 10-15minutes until golden brown. Remove from the oven and allow to cool on a wire rack.
To make the filling, whisk together the mascarpone, cream, icing sugar and vanilla until smooth and thoroughly combined.
Once pastry cases are completely cooled, spoon creamy filling into each case and top with strawberries.