1 For the crumble: place the flour, xanthan gum, salt, sugar and brown sugar in a mixing bowl. Whisk to combine.
2 Add the oats, and mix to combine. Create a well in the centre of the dry ingredients and add the melted butter. Mix to combine. Chill while you make the cake.
3 Heat the oven to 180°C (160° fan) 350°F gas 4. Grease a deep 23cm |9" cake tin and line the base with non-stick baking paper.
4 Place the apples, ground cinnamon and brown sugar in a heavy-based pan and toss to coat the apples in the cinnamon and sugar. Add 2 tablespoons of water and mix to combine.
5 Cook, stirring occasionally, until the apple slices are fork-tender adding more water if necessary to prevent the mixture from burning. Remove from the heat and set aside to cool.
6 Place the flour, xanthan gum, baking powder and salt in a bowl and whisk to combine. Set aside.
7 Beat the butter in a mixing bowl until light and fluffy. Add the sugar, then the eggs and vanilla, beating to combine well after each addition.
8 Add the dry ingredients in 3 parts, alternating with the milk and beginning and ending with the dry ingredients. Mix until just combined. Fold in the cooked apples.
9 Spoon evenly into the tin.
10 Break the chilled crumble into loose, irregular pieces. Place the crumble pieces on top of the cake mixture in an even layer and press down gently.
11 Bake for about 45 minutes, until the crumble topping is lightly golden brown. Remove from the oven and cool in the tin for at least 30 minutes. Sift a little icing sugar over the top.