Peel the cooking apples, and cut into bite-sized chunks. Add them to a baking dish, and sprinkle with sugar, cinnamon and nutmeg. Mix to combine, and set aside.
Add the diced butter and plain flour to a mixing bowl. With clean hands, rub together between your fingertips until the mixture resembles coarse breadcrumbs. Add the granola and a little more sugar, and mix to combine.
Scatter the crumble mixture over the apples, and bake at 190°C (Gas Mark 5 / 375°F) for around 45 minutes, or until the apples are soft and the crumble topping is golden and crispy.