Peel, core, and dice the apples into small pieces
Place a saucepan on medium heat and melt 1 Tbsp. butter and ¼ cup sugar. Add the diced apples, lemon juice, 1 Tbsp. water and apple pie spice. Let it cook for about 5-7 minutes, stirring occasionally
In a separate bowl, mix 1 tablespoon of cornstarch with 1 tablespoon of water to create a slurry. Stir until it's smooth and there are no lumps.
Slowly pour the cornstarch slurry into the simmering apple mixture while stirring constantly. This will help thicken the filling.
Let the apples simmer in the filling for about 5-10 minutes or until they are tender but not mushy. Stir occasionally to ensure even cooking.
Remove the saucepan from the heat, cover with plastic wrap, and let the apple pie filling cool.