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Put the slices of apple in a slow cooker. Sprinkle over the ground cinnamon, orange zest and 1 tbsp juice and mix together.
Put the oats and walnuts into a food processor and pulse a few times to give the texture of very course breadcrumbs. Pour into a bowl.
Stir the ginger, flour and sugar into the oat and walnut mixture, then add the butter and mix well.
Spoon the crumble mix over the apples, covering them all. Lay two sheets of kitchen paper on top of the crumble. Cover with the slow cooker lid and cook on a low heat for 3½ hours.
Remove the kitchen paper and cook for the last 10 minutes with the lid just slightly ajar.