Prepare the Apples: Start by peeling, coring, and slicing the Granny Smith apples into thin wedges.

Mix and Marinate: Toss the sliced apples in a large bowl with granulated sugar, ground cinnamon, fresh lemon juice, and a pinch of nutmeg if using. Let sit for 15 minutes.

Create the Thickening Agent: In a small bowl, mix the flour or cornstarch with a splash of water until smooth.

Cook the Filling: Transfer the apple mixture to a large skillet and cook over medium heat for about 10 minutes, stirring gently.

Add the Butter: Stir in the melted butter, and let the mixture cool before using.