01 - Set the oven to 350°F and allow it to reach temperature.
02 - In a large bowl, combine apples, pears, cranberries, brown sugar, cinnamon, nutmeg, lemon juice, and cornstarch; toss until fruits are fully coated.
03 - Transfer the fruit mixture into a greased 9-inch baking dish and spread evenly.
04 - In another bowl, blend flour, rolled oats, brown sugar, cinnamon, and salt.
05 - Rub cold cubed butter into the dry mixture using fingertips or a pastry cutter until coarse crumbs form; fold in nuts if using.
06 - Distribute the crumble topping evenly over the fruit layer.
07 - Bake for 35 to 40 minutes until the topping is golden brown and fruit filling is bubbling.
08 - Let rest slightly before serving warm, optionally accompanied by vanilla ice cream or custard.