What to do
1. Preheat the oven to 180°C
2. Peel the pears and apples if desired, chop them into bite-size chunks, and place them in a 9-inch/23 cm baking dish. Add the blackberries and the lemon juice, and toss to coat.
3. Sprinkle the sugar, arrowroot, and 2 teaspoons of the cinnamon over the fruit, and toss to mix.
4. In a bowl, combine the rolled oats, maple syrup, walnuts, the remaining 1 teaspoon cinnamon, cardamom, flour, and salt. Cut in the cold coconut oil until the mixture is crumbly. Spread it over the fruit.
5. Bake the crumble for 25 to 30 minutes, until the fruit is bubbling and the top is golden brown. Let it cool slightly, and serve warm with yogurt or ice cream if desired.