Preheat the oven to 400 degrees F.
Peel, core and slice the apples, then peel, core and grate the quince. Fill a pie pan with the fruit, sprinkling sugar over each layer, and mounding it up toward the center of the pan. Pour the water into the pan to come halfway up to the fruit.
Roll out the dough. Cut off a thin strip and attach it to the rim of the pan, brushing the rim with water first. Brush the strip with water and cover the pan with the sheet of dough, cutting off all the overhanging bits. Crimp the edges, brush the top with eggwash, and strew over the demerara sugar. Bake for 20 minutes before turning the temperature down to 375 degrees F and cooking for a further 20 minutes. Eat warm or hot with heavy cream.
Shortcrust Pastry dough:
Use approximately twice the weight of all purpose flour (preferably organic) to unsalted butter. Some recipes call for half butter, half lard.
Sift the flour and a pinch of sea salt into a food processor, then cut the cold butter into small pieces on top of it. I process it for 20 to 30 seconds, then add ice-cold water through the top, a tablespoon at a time, 2 to 2 1/2 minutes should be enough for about 10 ounces of dough, with the machine running. If the paste is still in crumbly little bits after 1 or 2 minutes, add a tablespoon more water, but remember, the more water you use, the more the crust will shrink if you bake it blind. One solution is to use a bit of cream or egg yolk instead of water. The moment the dough has cohered into a single ball, stop, remove it, wrap it in plastic wrap and chill in the fridge for at least 30 minutes.
If you're making pastry dough by hand, sift the flour into a large bowl with the salt, add the chopped butter, and work as briskly as you can to rub the fat into the flour. Use the tip of your fingers only, rather like running grains of hot sand through your fingers. Add the water bit by bit as above; wrap and chill the dough.
If you're making a double crust pie, divide the dough into roughly 2/3 and 1/3. Then scatter a bit of flour on your work surface, roll your rolling pin in it, dust the palms of your hands, and start rolling. Always roll away from yourself, tuning the dough as you go, and keep the rolling pin and work surface floured to prevent sticking.
Preheat the oven to 375 to 400 degrees F.
Line your greased pie pan with dough. Never stretch it; it will stretch back. Try to leave at least 30 minutes for the unbaked dough to commune with the inside of your fridge. Or put it in the night before you need it.
Tear off a piece of waxed paper a little larger than the pie pan and place it over the dough. Cover the paper with a layer of dried beans; the idea is to prevent the pastry from rising up in the oven. When the dough is nearly cooked (the timing depends on the rest of the recipe), remove the paper and beans and prick the bottom of the pie shell to let out trapped air that would otherwise bubble up. Return the tart to the oven for 5 to 10 minutes to dry the bottom. Brushing the partly baked pie shell with a light coating of beaten egg or egg white ensure a crisp finished tart.
Sift the flour and salt into a mixing bowl. Rub in 2 tablespoons of the butter, as for shortcrust pastry dough, or use a food processor. Mix in the water and then gently knead the dough on a floured surface, preferably marble. Wrap it in plastic wrap and refrigerate for 30 minutes.
Keep the rest of the butter out so that it softens, then flatten it into a rectangle 1 inch thick. On a lightly floured surface, roll out the dough into the same size as the butter. Place the butter in the center of the pastry and then fold over the top and bottom of the dough to cover the butter.
With the rolling pie, press down on the edges to seal in the butter, then give the dough a quarter turn clockwise. Now roll the dough out so that it returns to it's original length. Fold over the ends again, press them together with the rolling pin, and give a further quarter turn clockwise. Repeat the process once more, then rest the dough in the fridge for at least 30 minutes, remembering which way it is facing.
Repeat the rolling and turning process twice more, then refrigerate for a final 30 minutes before using or freezing. If the dough gets warm and buttery at any stage during the process, put it in the fridge to chill.
If you prefer not to make your own, you can buy ready made puff pastry, but try to find the very best available.