In a medium saucepan over medium-high heat, combine the sliced apples and water. Cook for 6 minutes, stirring occasionally. Add the butter and allow to melt.

In a separate bowl, whisk together the sugar, cinnamon, cornstarch, and nutmeg.

Add the sugar mixture to the saucepan. Stir constantly for 2 minutes, or until the sauce thickens.

Remove from heat and allow the filling to cool.

Store the cooled filling or use it as directed in the recipe of your choice.