Prepare the peaches: If using fresh peaches, peel, pit, and slice them into thin wedges. If using frozen peaches, allow them to thaw slightly and drain any excess liquid.

1. Cook the peaches: In a medium saucepan over medium heat, combine the sliced peaches, granulated sugar, lemon juice, and vanilla extract. Stir gently to combine. Bring the mixture to a simmer, stirring occasionally, until the peaches start to release their juice, about 5-7 minutes.
2. Thicken the filling: In a small bowl, whisk together the cornstarch and water until smooth. Add the cornstarch mixture to the simmering peach mixture while stirring. Continue cooking and stirring for another 2-3 minutes, or until the filling thickens to a pie-filling consistency. If you like, add ground cinnamon at this point for an extra layer of flavor.
3. Cool the filling: Remove the saucepan from the heat and allow the peach filling to cool for about 15-20 minutes before using it in your pie or dessert. This helps the filling to set and thicken even more.
4. Use or store: Once cooled, the peach pie filling is ready to use. If you’re not using it immediately, you can store it in an airtight container in the refrigerator for up to 3 days.