Preheat oven to 200°C. Line a large baking sheet with baking paper. Chill a large mixing bowl in the freezer for at least 15 minutes while you prepare the crumble.
1. In a bowl combine the flour, oats, brown sugar and salt. Cut in the butter until the mixture resemble coarse crumbs. Stir in the nuts, scatter onto the baking sheet and bake for 15 minutes or until golden. Stir to crumble the mixture and leave to cool.
2. Line a 30 x 20cm rectangular dish with clingfilm. Spread half the crumble on the bottom.