Place the cottage cheese in a bowl. Add the egg, sweetener and half of the cinnamon, then mix until well combined.
Stir in the ground almonds or oats, followed by the chopped chestnuts or walnuts.
Transfer the mixture to a shallow, lightly greased oven-proof dish.
Peel the apple, remove the core, then cut the flesh into small cubes.
Scatter the apple pieces over the cottage cheese mixture and dust with the remaining cinnamon.
Bake in a preheated oven at 180°C (or in an air fryer) for about 15 minutes, until set and lightly golden on top.
Serve warm or at room temperature, drizzled with extra honey or syrup if desired.