Peel and slice the Granny Smith and Honeycrisp apples into even pieces, about 1/4 inch thick. Toss them with fresh lemon juice to prevent browning.

In a large bowl, combine the granulated sugar, ground cinnamon, ground nutmeg, and cornstarch. Stir well to create a fragrant sweet spice mix.

Gently fold the sliced apples into the sugar-spice mixture until they are evenly coated.

In a medium saucepan over medium heat, add the apple mixture. Cook while stirring occasionally until the apples are soft, about 5-7 minutes.

Once the apples are softened, stir in the cubed butter until it melts and combines.

Remove the saucepan from the heat and let the filling cool slightly before using.