Place all the apples in a saucepan then add half the brown sugar and a little water. Put on the lid then bring to the boil. Reduce heat immediately to a simmer and leave for 15 minutes. Remove the lid and stir often until liquid evaporates. Cool.
Preheat oven to 180 degrees C. Grease and line a 30 x 20 cm lamington tin with baking paper.
Place flour, icing sugar and butter in a food processor then pulse until crumbly. Add 3 tablespoons of iced water then pulse until mixture forms small clumps. (Add 1 more tablespoon of water if needed).
Place 1/3 crumble mixture in a bowl; cover and chill, knead remaining mixture into a dough: press evenly in base of the prepared tin.
Line pastry with baking paper then bake blind for 10 minutes. Remove weight and paper then bake for 10 minutes more or until pale golden. Cool.
Add the oats, walnuts, cinnamon and remaining brown sugar to chilled crumble. Rub in 30g butter to make coarse crumbs.
Over the prepared base, spread the cooled apples. Sprinkle the crumble mixture over apples. Bake for 20 minutes or until golden. Cool slightly before cutting.