- With the rack in the middle position, preheat the oven to 350°F (180°C). Butter the sides of an 8-inch (20 cm) springform pan with removable bottom and line the bottom with parchment paper.
- In a bowl, combine the oats, flour and brown sugar. Add the butter and combine just until the dry ingredients are moistened. Set aside.
- In a bowl, combine the apples, brown sugar and lemon juice. Set aside.
- In a bowl, combine the flour, cinnamon and baking soda.
- In another bowl, cream the butter and brown sugar with an electric mixer. Add the eggs, one at a time, and beat until combined. With the machine on low speed, add the dry ingredients alternating with the sour cream until smooth. Pour into the springform pan. Drain the apple filling slightly and arrange over the cake batter in the pan. Top with the crumble.
- Bake for 1 hour or until a toothpick inserted in the centre of the cake comes out clean. Let cool slightly before unmoulding. Let cool completely on a wire rack.
- Once the cake has completely cooled, generously dust with icing sugar. Drizzle with the caramel before serving.