Preheat the oven to 160° Celsius

Using a food processor, combine the butter, flour and sugar until they resemble fine bread crumbs

Add the oats and nuts and blitz until mixed

Add the lemon zest and mix in by hand

Butter your ovenproof dish and place the apples evenly over the bottom

Sprinkle generously with the cinnamon sugar

Make a think covering of the crumble over the apples and press down

Now add a second layer

Bake for 30 minutes