- With the rack in the lowest position, preheat the oven to 180 °C (350 °F).
- In a bowl, combine the oats, flour, and brown sugar. Add the butter and stir to combine. Set aside.
- In another bowl, combine the brown sugar and cinnamon. Add the apples and lemon juice and stir to combine.
- Spoon into a 20-cm (8-inch) square baking dish. Cover with the topping. Bake for about 50 minutes. Let cool.
- Serve warm or at room temperature. Delicious with vanilla ice cream.