Peel, core and chop the apple into 2cm pieces.
Preheat the oven to gas 4, 180°C, fan 160°C. In a large bowl, mix the fruit with the cinnamon, 1tbsp of the flour and 1tbsp of the brown sugar. Pour into a small baking dish.
In a medium bowl, combine the remaining brown sugar, flour, nuts, butter and oats. Use your fingers to work the butter through but keep it chunky. Scatter the topping over the fruit and bake for 45 minutes until golden. Serve warm, with ice cream or crème fraîche.