Preheat your oven to 190C / 375F / Gas mark 5 and get your pie dish out. If you are not using a non stick loose bottom pie dish then lightly grease your pie dish.
Roll out your pastry to a 10 inch round and lay it in your pie dish. Gently pushing the pastry into all the edges of the dish. Leave a little overhang on the edges of your pastry as it will shrink a little while baking.
Cover the pastry with baking parchment and place ceramic baking beans or uncooked rice over it to weigh it down while you blind bake your pastry case.
Pop it into your oven for 10 minutes.
While your pastry is baking, wash & top and tail your rhubarb stalks then cut them into slices about half an inch thick and pop them into a bowl and sprinkle the sugar over it. Give the rhubarb a toss in the sugar so it is all covered and place it to one side.
After 10 minutes take your pastry shell out of the oven, remove the ceramic baking beans and using a sharp knife trim off any excess from the pie edges then place it to one side.
Drain off any liquid that has gathered in the bottom of the bowl with the rhubarb and add in the tin of cherry pie filling and give it a good mix.
Pour the fruit into the pie shell and if you wish to with the off cuts of pastry shape them into a cherry and place it in the middle of the pie and then pop it into the oven for 15 to 20 minutes until the fruit filling is lightly bubbling.
Take it out of the oven and leave it for 5 minutes to slightly cool before serving.
This pie can be enjoyed hot or cold. Once it has fully cooled down keep it refrigerated and eat within 3 days.