Preheat oven to 180°C / 350°F (both fan and conventional ovens are fine).
Prepare the apples by peeling and chopping them into small cubes, roughly 1.5cm / 1/2 inch in size.
Make the filling by combining the apples with flour, sugar, and cinnamon in a bowl. Drizzle with lemon juice or water, then gently toss everything together. Transfer the mixture to a 1.5-litre (or 1.5-quart) baking dish, spreading it evenly.
Create the topping by mixing the oats, flour, sugar, baking powder, cinnamon, salt, and melted butter in a separate bowl. Stir until a crumbly texture forms, resembling wet sand. If needed, use your fingers to break the mixture into smaller clumps. Scatter this over the apple mixture.
Bake in the preheated oven for 30-40 minutes until the topping is golden and crisp. Once done, remove from the oven and loosely cover with foil to keep it warm. Let it rest for about 10 minutes so the apple juices can thicken.
Serve warm, paired with a scoop of vanilla ice cream for the perfect finishing touch.