Preheat oven to 180°C (350°F).
Peel and slice apples, then combine with diced beetroot in a large baking dish.
Add lemon juice, golden caster sugar, water, arrowroot powder, cinnamon, vanilla extract, ginger, and nutmeg to the fruit mixture. Toss everything together until well coated.
In a separate bowl, combine rolled oats, almond flour, brown sugar, melted coconut oil, chopped walnuts, and cinnamon for the topping.
Spread the crumble topping evenly over the fruit filling.
Bake for 45 minutes until the top is golden brown and the filling is bubbling around the edges.
Allow to cool for 10 minutes before serving warm.