Before cooking, wash your hands and put an apron on. Weigh or measure all the ingredients. Put some baking paper on the baking tray. Ask a grown-up to preheat the oven to 200C/400F/Gas 6.
Flour your worktop or table with a little plain flour. Shape the pastry in your hands into a rough disc shape. You can turn it on its side and roll it or use the palm of your hands to pat the corners until it is round.
Roll the pastry out, turning the pastry often so that it stays round. It should be about 5mm thick and about 20cm across. Put onto the baking tray.
Sprinkle the centre of the pastry with the oats, leaving space all around – like an island of oats – these will soak up the fruit juices!
Pull the stalks off the cherries and put them into a plastic bag. Bash gently with the rolling pin to split them. Tip the cherries and their juices into the mixing bowl and pick out the stones – be sure to double check that all the stones have been removed. Add one tablespoon of the demerara sugar and a pinch of cinnamon, then stir.
Place the cherries into the middle of the pastry on top of the oats, leaving the edge of the pastry free of fruit. Brush the edge with cherry juice.
Now bring the edges of the pastry up over the cherries and pinch the pastry together to hold it in place. You should still be able to see some of the fruit at the top of the pie. Now brush the pastry with the juice again. Sprinkle a little more sugar on top.
Ask a grown-up to put it in the preheated oven for 20–25 minutes. Serve warm or cold with yoghurt or ice cream.