Preheat the oven to 180°C (gas mark 6) and grease a cake pan.
Arrange the apple cubes on the bottom of the pan and sprinkle lightly with sugar.
Add the yogurt, milk, vanilla extract, and lemon zest (if using) and mix well.
Pour the mixture over the apples in the pan.
Bake for about 40-45 minutes, or until the flan is golden and firm.
Let it cool slightly before removing from the pan and serving this delicious apple and yogurt flan. Enjoy it hot or cold.
