Preheat the oven to 170C/325F/Gas 3.
Mix the cinnamon with the sugar in a bowl. Place half the apples in the ovenproof dishes and sprinkle with half of the sugar mixture. Cover with the remaining apples and add the remaining sugar and cinnamon mixture.
To make the crumble topping, place the flour and cinnamon in a large bowl and mix well, then stir in the sugar and the oats. Cut the butter into small cubes, add to the mixture and rub it in with your fingertips until the mixture is the texture of coarse breadcrumbs.
Arrange the topping over the fruit and bake the crumbles for about 40 minutes or until the topping is golden and crunchy. Serve with custard.