Add the cherries, sugar, lemon juice, cornstarch, and water to a 1 quart saucepan. Stir until the cornstarch is fully dissolved.
Place the saucepan over medium heat and cook, stirring frequently, until the mixture comes to a boil.
Reduce the heat to low and simmer for about 5 minutes, stirring occasionally, until the cherries soften and the filling thickens and coats the back of a spoon.
Remove from the heat and let the cherry pie filling cool to room temperature. It will continue to thicken as it cools.
Transfer to a jar or bowl and refrigerate until ready to use.Keep covered in the refrigerator for up to one week.
Notes
Use Ripe or High-Quality Cherries: Fresh, ripe cherries give the best flavor. If using frozen cherries, thaw and drain excess liquid to prevent a thin filling.
Cook Until Thick and Glossy: Simmer the cherry pie filling until it bubbles and looks thick and shiny. The mixture should coat the back of a spoon. It will thicken more as it cools.
Do Not Skip the Boil: Cornstarch must reach a full simmer to activate properly. If the filling does not boil briefly, it may not thicken.
Cool Before Using: Let the filling cool completely before adding it to a pie crust, crisp, or cheesecake. This helps it set properly and prevents a soggy crust.