Whisk the sugar and cornstarch together until well combined.
Add the cherries, sugar mixture, and 1/4 cup of water to a medium large non-reactive pan. Stir to combine and heat over medium heat, stirring constantly to dissolve the sugar and cornstarch. As the cherries heat they will start to release their juices.
Once the mixture boils, reduce the heat and continue to simmer and stir for a few minutes until the filling has thickened to your desired consistency. Keep in mind your filling will thicken further as it cools.
I like to taste the filling (be careful, it's hot!) to see if it needs any more sugar, or a squeeze of lemon.
Let the filling cool to room temperature and then refrigerate in an airtight container until needed.
Notes
This recipe will fill a 9 inch pie.
If you use sweet cherries you might need to reduce the sugar and increase the lemon juice.