Peel and slice the apples into thin wedges. Aim for uniform thickness to ensure even cooking.

In a large mixing bowl, combine the sliced apples with lemon juice. This prevents browning and adds a fresh flavor.

Add granulated sugar, ground cinnamon, ground nutmeg, salt, and flour to the apples. Stir gently to coat the apples evenly with the dry ingredients.

Let the mixture sit for about 10 minutes. This allows the apples to release their juices, which will create a flavorful filling.

In a medium saucepan over medium heat, melt the butter. Once melted, add the apple mixture to the saucepan.

Cook for about 5-10 minutes, stirring occasionally, until the apples are slightly softened but still hold their shape. This step enhances the flavors and helps thicken the filling.

Remove from heat and let cool before using. The filling can be used immediately or stored in the refrigerator for up to 3 days.