Preheat the oven 160°C (325°F) and line a baking tray with baking paper.
Spread the walnuts out over the baking tray and place in the oven for 10-15 minutes to roast them. They should be lightly golden.
Remove from the oven and set aside to cool.
Grease and line a 9"x13" baking pan and set aside.
Prepare the base by mixing the melted butter, maple syrup, milk, vanilla extract, eggs, caster sugar and brown sugar in a bowl until thick and combined.
Add in the self-raising flour and cinnamon and mix. Pour into the prepared dish and set aside.
Combine the roasted walnuts, oats, flour, cinnamon, nutmeg and brown sugar in a separate bowl and mix together. Set aside.
Peel, core and dice the apple into small chunks or thin slices. Which ever you prefer. (I do a mixture of both).
Place in a bowl with the maple syrup, cinnamon and mixed spice and mix.
Spread in an even layer over the base, pressing some of the apple slices into the base.
Top with the crumble and finally pour over the remaining maple syrup and place in the oven for 45-55 minutes. The top and sides will be golden and it will be firm. A skewer inserted in the center should come out clean.
Allow to cool before slicing and serving.
Best kept in the fridge for up to a week as the apples will go soggy in the heat.