To make the pastry, sift the flour and salt into a large mixing bowl, holding the sieve up as high as possible to give the flour an airing.
Then cut the fat into small cubes and add to the flour. Now, using your fingertips, lightly and gently rub the pieces of fat into the flour – lifting your hands up high as you do this (again to incorporate air) and being as quick as possible. When the mixture looks uniformly crumbly, start to sprinkle roughly 2 tablespoons of cold water all over. Use a round-bladed knife to start the mixing, cutting and bringing the mixture together. Carefully add more water if needed, a little at a time, then finally bring the mixture together with your hands to form a smooth ball of dough that will leave the bowl clean (if there are any bits that won't adhere to it, you need a spot more water).
Now rest the pastry, wrapped in foil or polythene, in the refrigerator for 20-30 minutes before rolling out. Roll out the shortcrust pastry and line the flan tin.
For the filling, halve the plums and remove the stones. Now beat the soured cream together with the caster sugar, egg yolks and mixed spice, then pour this into the pastry case and arrange the plums over the top (flat side up). Place the flan on the baking sheet and bake for 20 minutes. Then mix the cinnamon with the demerara sugar and sprinkle it all over the top. Bake for a further 20 minutes, turning the heat right up to gas mark 8, 450°F (230°C) for the final 5 minutes so that the top can brown nicely.
Serve warm or cold.
Pre-heat the oven, and a baking sheet, to gas mark 6, 400°F (200°C).
Using a fan-assisted oven? Click here