Adjust the oven rack to the middle position and pre-heat the oven to 350ºF (180ºC). Lightly butter a 9-inch (23cm) round cake pan and line its bottom with a round of parchment/baking paper. Generously butter the parchment/baking paper as well.

Sprinkle the sugar in an even layer on the bottom of the prepared cake pan.

Arrange blueberries on top of the sugar so that they completely cover the bottom of the pan in a single, even layer. Make sure that they're packed together as tightly as possible.

Add the sugar and lemon zest to a large bowl (or the bowl of a stand mixer, if using), and use your fingertips to rub the zest into the sugar.

Add the softened butter, oil and vanilla, and cream everything together until pale and fluffy.

Add the eggs, one at a time, mixing well after each addition, until well combined.

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

Add half of the dry ingredients to the butter-sugar mixture, and whisk well to combine. Add the yoghurt and whisk until smooth. Finally, add the rest of the dry ingredients and whisk well until you get a smooth cake batter with no flour clumps.

Dollop the cake batter on top of the blueberries and smooth it out into an even layer.

Bake at 350ºF (180ºC) for about 45-50 minutes or until the cake is golden brown on top, well risen and an inserted toothpick or cake tester comes out clean.

Allow the cake to cool in the cake pan for about 15 minutes, then carefully invert it onto a large plate while it's still hot.

Serve the cake either warm or cooled completely to room temperature.