1-Prep: Prep. Preheat the oven to 375°F / 190°C / gas mark 5. Grease a 9-in / 23 cm / 2-quart baking dish or casserole dish with butter.
2-Make the apple filling: Make the apple filling. Add the cored and sliced or diced apples, brown sugar, ground cinnamon, and lemon juice into the baking dish. Toss everything together until the apples are evenly coated in sugar and cinnamon. Arrange in an even layer. Set aside.
3-Mix the crumble: Mix the crumble. To make the crumble, combine the rolled oats, gluten free flour, brown sugar, and ground cinnamon in a large mixing bowl. Add the melted butter and stir with a fork until the mixture becomes crumbly. Press handfuls of dough to former larger clumps.
4-Assemble the apple crumble: Assemble the apple crumble. Sprinkle the crumbs evenly over the apple mixture.
5-Bake: Bake. Bake for 40 to 55 minutes, until the topping looks golden brown and the apples are soft and bubbling. If the crumbs brown too quickly, cover with aluminum foil.
6-Serve: Serve. Serve the apple crisp warm or at room temperature with vanilla ice cream or caramel sauce.