Toss apple slices with 1 cup brown sugar, 2 tablespoons flour and 1 teaspoon cinnamon in a medium-sized baking dish. Top with 85g butter.
Make topping
Mix 2 cups flour, oatmeal, 2 cups brown sugar, 1 teaspoon cinnamon, bicarb, baking powder and salt in a large bowl. Rub in 225g butter until mixture is crumbly. Sprinkle over apples.
Bake in the preheated oven until bubbly and golden brown, about 45 minutes.
Cook's Note:
Add chopped nuts to the crumble topping if desired.