- With the rack in the middle position, preheat the oven to 350°F (180°C). Butter a square 8-inch (20 cm) baking dish and line with parchment paper, letting it hang over two sides.
- In a bowl, cream the butter and brown sugar. Stir in the flour until crumbly. Set aside 1/2 cup of the dough for the filling. Press the remaining dough into the bottom of the baking dish. Bake for 10 minutes.
- Meanwhile, in a bowl, crumble the oats and butter with the reserved dough.
- In a large bowl, combine the brown sugar, cornstarch and cinnamon. Add the apple slices and toss to coat well. Spread over the crust and sprinkle with the oat mixture.
- Bake for 45 minutes or until the top is golden brown. Let cool on a wire rack before unmoulding.