Adjust the oven rack to the middle position, pre-heat the oven to 350ºF (180ºC) and line a 9-inch (23cm) square baking pan with parchment/baking paper.Tip: When lining the baking pan, I recommend leaving some parchment/baking paper overhang, as that will help with removing the cake from the pan later on.

Make the blueberry filling first, so it has time to cool down while you prepare the other elements. The filling needs to be lukewarm or at room temperature when you assemble the cake.

Combine the blueberries, sugar and lemon juice in a saucepan, and cook them over medium-low heat until the blueberries have released their juices. At this point, the juices should be warm or lukewarm (and definitely NOT boiling!) and the blueberries should be soft but not mushy.

Stir in the cornstarch (US)/cornflour (UK) and mix well until it's evenly combined with the blueberry filling – you shouldn't see any lumps of cornstarch remaining.Tip: It’s VERY important that your blueberry juices aren’t hot or boiling when you add the cornstarch – that would make it clump up and give your blueberry filling a rather unpleasant texture. Instead, you want the juices to be warm at most when you add the cornstarch.

Increase the heat to medium-high and cook with frequent stirring for a further 3-4 minutes until the blueberry juices have thickened.

Remove from the heat, pour the thickened blueberry filling into a bowl and allow to cool while you prepare the streusel topping and the lemon cake batter.

Add the sugar and lemon zest to a large bowl, and use your fingertips to rub the zest into the sugar.

Add the softened butter and vanilla, and mix well with a wooden spoon or rubber spatula until combined.Tip: Don’t cream or aerate the mixture, as that can make the streusel too delicate and crumbly (note the absence of any raising agents for the same reason).

In a separate bowl, whisk together the gluten free flour blend, almond flour, xanthan gum and salt.

Add the dry ingredients to the butter-sugar mixture and mix well until it starts clumping together. Then, give it a knead until you get a homogeneous cookie dough. Set aside until needed.

Add the sugar and lemon zest to a large bowl, and use your fingertips to rub the zest into the sugar.

Add the buttermilk, melted butter, oil, eggs and vanilla, and whisk well until combined.

In a separate bowl, whisk together the gluten free flour blend, almond flour, baking powder, baking soda, xanthan gum and salt, and add them to the wet ingredients.

Whisk well until you get a smooth cake batter with no flour clumps.

Transfer the lemon cake batter into the lined baking pan and smooth it out into an even layer.

Spoon the cooled blueberry filling over the batter, making sure you get a fairly even layer with only a small amount of batter peeking through here or there. (See blog post for photos.)

Crumble the streusel topping over the blueberry layer, making sure that you get a fairly even coverage.

Bake at 350ºF (180ºC) for about 40 minutes or until the crumble topping is evenly golden brown and an inserted toothpick/cake tester comes out clean. Don’t worry if your toothpick comes out covered in blueberry juices, that’s to be expected – you just don’t want to see any half-baked batter or too many moist crumbs.

Allow the cake to cool until warm before you remove it out of the baking pan. While it’s hot, the cake will be fairly delicate, so I don’t recommend removing it out of the pan immediately out of the oven (it could crack or crumble).

Serve warm or cooled completely to room temperature. If you wish, you can dust the top with some powdered/icing sugar before slicing and serving.

The gluten free blueberry crumb cake keeps well in a loosely covered container in a cool dry place for about 3-4 days. (Keeping the cake in a tightly closed or air-tight container would make the crumble topping too soft.)