In a medium saucepan over medium-high heat, combine the sliced apples and water. Cook for about 6 minutes, stirring occasionally, until the apples begin to soften.
Add the butter and stir until fully melted.
In a medium bowl, whisk together the granulated sugar, ground cinnamon, cornstarch, and nutmeg until well combined.
Add the sugar mixture to the saucepan. Stir constantly for about 2 minutes, or until the sauce thickens and coats the apples.
Remove from heat and let the filling cool completely.
Use immediately in your favorite recipe or store the cooled filling in an airtight container in the refrigerator for up to 5 days.