Place the Mix in a bowl, cut the fats into small pieces using a knife and add them to the Mix.
Using your fingertips, rub the fats into the Mix until the mixture resembles coarse breadcrumbs. 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. 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. Divide pastry and use to line 4-6 loose based tart tins.
Blind bake in a preheated oven for 10minutes (200°C/180°C Fan/400°F/Gas Mark 6) until lightly golden.
For the filling, soften the onions in a pan over a low heat for approx. 10 minutes until golden and soft.
Beat together the egg and cream and season with freshly ground black pepper.
Divide the cheeses between pastry cases, add the onions, and pour cream mixture over the top.
Return to the oven and bake for a further 15-20minutes until filling is golden brown and set.