Preheat the oven to 175 degrees Celsius. Spread the walnuts on a baking tray in a single layer and bake them for 8-10 minutes until toasted. Let the nuts cool completely.
Place the walnuts and 10 grams (1/8 cup) instant oats in a food processor and pulse several times until the mixture resembles fine crumbs. Transfer the mixture into a mixing bowl and add in the remaining oats, flour, sugar and cold butter. Rub the butter into the dry ingredients (using a pastry blender, fork or your fingers) until the butter has been fully incorporated. Refrigerate the crumble mix until hardened, about 45 minutes to 1 hour.
Apples
Place the apples, cranberries, flour and cinnamon in a large mixing bowl. Toss to coat the apples and cranberries with flour and cinnamon. Add in the maple syrup and lemon juice and toss to combine. Leave aside for the flavours to mingle for 15 minutes.
Divide the apple mix between 4 ramekins. Divide the crumble into 4 portions, and crumble them on top of the apple mix, pressing down gently.
Bake for 30 minutes until the apples are tender, the juices are bubbling thickly and the topping is golden and crisp. Serve warm or at room temperature, dusted with icing sugar.
Make ahead: You can refrigerate the apple mix for a couple of hours. The addition of lemon juice will prevent the apples from turning brown.