In a large pan heat the cherries and add the sugar and lemon juice. Allow the cherries to release their juice.
In a small bowl whisk together the cornstarch and water and then pour into the pan with the cherries. Stir everything together, let it bubble and thicken then take off the heat and allow to cool.
Meanwhile, make the pie dough (shortcrust pastry). In a food processor add the flour, butter, sugar and salt. Pulse until the mixture resembles cornmeal - crumbly and slightly yellow from the butter.
Transfer the mixture to a bowl and add the water, start mixing with a spoon then use your hands to bring the dough together to form a rough ball. Wrap with clingfilm and refrigerate for around an hour. You may need to repeat this step/make a larger dough if you are using a big baking tray.
Preheat the oven to 180°C/360°F. Roll out the dough and press it into a greased shallow baking tray - the sides shouldn't be too high so you can easily get a slice out. Make sure the dough overhands the edges slightly as it'll shrink slightly during cooking.
Pour in the filling and cut out strips from the remaining dough to make a lattice.
Line up the strips of dough along one side of the pie (but don't press them in yet).
Pull back every other strip half way and lay a strip at 90° in the middle. Bring back the strips then fold back the others and lay another strip at 90° - see video for a better explanation!
Brush the pastry with egg wash and sprinkle with demarara.
Bake for around half an hour until golden and bubbling. Serve warm with ice-cream.