Prepare the Apple Filling:
Preheat oven to 180°C (fan).
In a saucepan, combine diced apples, caster sugar, and mixed spice.
Cook over medium heat for 3-5 minutes until apples start to soften.
Transfer to an ovenproof dish and spread evenly.
Make the Crumble Topping:
In a mixing bowl, combine gluten-free flour and diced butter.
Rub butter into flour using fingertips until the mixture resembles breadcrumbs.
Stir in muscovado sugar, oats, and salt.
Assemble and Bake:
Evenly sprinkle the crumble mixture over the apple filling.
Bake for 35-40 minutes or until the top is golden and crisp.
Allow to cool slightly before serving.
Serving Suggestions:
Serve warm with vanilla ice cream, whipped cream, or custard.