Preheat the oven to 180°C / 350°F (suitable for both fan-forced and conventional settings). Peel the apples and dice them into approximately 1.5 cm (½ inch) cubes. In a large mixing bowl, combine the apples with the flour, sugar, and cinnamon, then drizzle with lemon juice. Mix thoroughly to coat the apple pieces evenly, then distribute them in a 1.5-litre (1.5-quart) baking dish.
For the topping, combine all the ingredients in a separate bowl. Stir until the mixture resembles damp sand and forms clumps; use your fingers to enhance the crumble texture if necessary. Scatter the topping evenly over the apple layer.
Transfer the dish to the oven and bake for 30 to 40 minutes, or until the surface achieves a golden brown hue. Once baked, remove from the oven, loosely cover with foil, and allow it to rest for 10 minutes to allow the syrup to thicken slightly.
Serve while warm, accompanied by vanilla ice cream.