Mix together the flour, cornstarch and salt in a large bowl. Cut the butter into ½ inch cubes and place on a plate. Mix the water and vinegar together in a jug. Place everything in the fridge for at least 15 minutes.
Place all the dry ingredients into the bowl of a stand mixer fitted with the paddle attachment. Add the butter and toss them in the flour, making sure they're completely covered in flour.
Put the speed to low and mix together until the butter is the size of peas, about 2 minutes. If using your hands, mix together the butter and flour until the size of peas.
With the mixer running on low, pour in the water/vinegar, until the mixture is wet enough - I didn't use all of the water and left about 1 tbsp. If using your hands, create a well in the center of the bowl and pour the water/vinegar there. Use your hands to toss the mixture together. You don't want it to be too wet or too dry.
Place the dough onto a floured surface and use your hands to shape it into a disc. Cover in plastic wrap and chill for at least 1 hour.
Preheat the oven to 400°F/200°C. Line a baking tray with foil to catch any bubbling juices.
After 1 hour, roll the dough out on to a lightly floured surface until it is about 1 inch wider than your pie plate. Carefully place the dough on to your pie plate. Tuck the excess under and press it gently together to make your edges. Crimp the edges as you wish, I used a fork and gently pressed it along the sides. Chill for 15 minutes while you prepare the filling
Cherry Rhubarb Filling + Topping
In a large bowl, mix together the cherries, sliced rhubarb and the remaining pie ingredients. Leave for 10 minutes.
In a medium bowl, mix together all the crumb toppings until it resembles wet sand.
Pour the filling into the pie plate and sprinkle over the topping. Chill for another 15 minutes.
Place the pie plate on the baking sheet and bake for 55-60 minutes. Make sure the juices are bubbling, that's how you know it is ready.
Notes
Storing: keep in an airtight container in the fridge for up to 5 days. Frozen Fruit: I used frozen fruit in this recipe. If you're also using frozen fruit, place them in a bowl in the fridge for at least 4 hours beforehand to thaw. Drawn any remaining liquid before you make the filling. Kosher Salt: If you don't have kosher salt, use half the amount of salt stated in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt.Butter: I used European style butter in this recipe which makes the pie dough extra flaky!