Place the sliced apples, lemon juice, raisins, demerara sugar, vanilla extract and cinnamon in a large bowl and mix it all gently together. Put aside to marinate.
Put the plain flour, caster sugar and mixed spice into a large mixing bowl. Add the butter into the flour rubbing until it resembles breadcrumbs. Add the egg yolk and use your hands to pull the mixture together to make a ball of pastry. You may need to add a small amount of water.
Place the pastry onto a lightly floured surface, gently roll into a rough circle, 30cm across, then gently lift it onto a baking tray, not worrying if it overlaps as you will be folding the edges over.
Place the apple mixture into the center of the pastry leaving a border about 10cm wide.
Fold over the sides, not worrying about any overlapping. Brush the pasty with the egg white and then sprinkle with a little more sugar. Cover the center hole with a little piece of aluminium foil so the apples and raisins don't burn!
Bake until the apples are soft and the pastry is golden brown, about 35 minutes.
Serve with custard or ice cream.
Don't try to make a perfect circle of pastry, its a quick and easy recipe with a rustic look.