Preheat oven to 350°F (180°C) and line a 8x4 (20 cm x 10 cm) baking pan with parchment paper.
Wash, peel, and core the apples. Then, cut them into quarters and slice thinly using a mandoline or potato peeler. Aim for slices around 0.05 inches (1.5 mm) thick. Set the sliced apples aside.
In a bowl, mix together the cream cheese and eggs until smooth. If desired, you can add 2 to 3 tablespoons of granulated sweetener, vanilla extract, cinnamon, and lemon zest at this stage. Stir everything well until combined.
Once the custard ingredients are well-mixed, gently fold in the thinly sliced apples. Make sure they are evenly distributed in the mixture.
Transfer the batter into the prepared baking pan, spreading it out into even layers.
Bake on the middle rack of a preheated oven for 40 to 50 minutes, or until the center is set and the top is golden brown. Let cool completely to room temperature before slicing and serving.