Preheat the oven to 350F/180C, peel the apple and chop it up into small chunks.
Place the apple in a saucepan alongside the white sugar and cinnamon and cook over a medium heat for around five minutes until golden and softened. Don't forget to stir regularly so the apple doesn't stick to the bottom of the pan.
When cooked, remove from the heat and stir the apple in with all the other ingredients. You're looking for a slightly damp consistency - there should be no powdery oats. Everything should combine and stick fairly well. If you think it's too dry, simply add more water 1 teaspoon at a time.
Bake for 40-50 minutes, or until golden and crisp. I was checking on mine every 10 minutes or so to make absolutely sure it didn't burn.
Leave to cool on the baking tray before storing in an air-tight container.