Wash, peel, and slice the apples around the core until you get three or four large apple pieces from each apple. Cut the large apple pieces into ½" slices. Dice those slices into small-sized pieces.
Place the diced apples, salt, lemon juice/zest, granulated sugar, and butter in a saucepan over medium to medium-high heat. Stir and cook for a few minutes.
Stir in the cinnamon and nutmeg. In a separate small bowl, whisk together the cornstarch and apple cider. Pour this slurry mixture in with the apple mixture. Stir this mixture and cook for another 10 minutes or until al dente. The apples should be cooked fully but they'll still have a little bit of a bite to them.
Once the apples reach that stage and the sauce is thicker, take the pot off the heat and add in the vanilla extract. Stir until combined. Pour the apple filling into a shallow dish, "goo" included.
Allow the mixture to cool at room temperature for 15 minutes. Cover in plastic wrap and allow it to cool in the fridge for at least one hour or until no longer warm.