Prepare the apples by peeling, coring, and chopping into chunky pieces, then place in a grill-proof pan. Sprinkle the caster sugar and cinnamon sugar (if using) over the apples.
In a bowl combine the flour, brown sugar, butter, and oats. Using your fingers, rub the butter through the mixture until you have a very coarse texture. Stir in the walnuts. Sprinkle the crumble topping evenly over the apples in the pan.
Set up your Q for indirect cooking and preheat for 10 minutes.
Place the pan with the apple crumble on the trivet and turn the burner control knob(s) to the roast/bake setting. Close the lid and cook until golden, 30 to 40 minutes.
Spoon the apple crumble into serving bowls. Serve with ice cream or cream.