Step 1: Preheat your oven to 190°C (375°F) and lightly grease a 9×13 inch (23×33 cm) baking dish.
Step 2: Prepare the fruit filling. In a large bowl, combine the diced apples, pears, cranberries, 50g light brown sugar, and ground cinnamon. Toss gently until the fruit is evenly coated. Pour the fruit mixture into the prepared baking dish and spread it into an even layer.
Step 3: Make the crumble topping. In a separate medium bowl, whisk together the all-purpose flour, 50g light brown sugar, rolled oats, and a pinch of salt. Add the cold, cubed unsalted butter to the dry ingredients.
Step 4: Using your fingertips or a pastry blender, work the butter into the dry ingredients until the mixture resembles coarse breadcrumbs. If using, stir in the chopped pecans or walnuts.
Step 5: Evenly sprinkle the crumble topping over the fruit mixture in the baking dish, ensuring the fruit is mostly covered.
Step 6: Bake for 40-45 minutes, or until the fruit filling is bubbling around the edges and the crumble topping is golden brown and crisp. If the topping browns too quickly, you can loosely cover it with foil.
Step 7: Remove from the oven and let the crumble cool for at least 15-20 minutes before serving. This allows the fruit filling to set slightly.