Adjust the oven rack to the middle position, pre-heat the oven to 350ºF (180ºC) and line a 9-inch square baking pan with parchment paper.

Make the strawberry filling first. Combine the strawberries, sugar, lemon juice, and vanilla in a large saucepan, and cook over medium heat, stirring occasionally, until softened and juices have been released, about 3-4 minutes.

Remove the strawberries from the heat. In a small bowl, mix cornstarch and water to create a slurry. Add this to the strawberries, stir well, and return to the heat for about 1 minute until thickened. Pour into a bowl or deep plate and cool to room temperature.

For the streusel topping, mix the sugar and lemon zest in a large bowl, then add the softened butter and vanilla, mixing until combined. In a separate bowl, whisk together the gluten free flour, almond flour, xanthan gum, and salt. Add dry ingredients to the butter-sugar mixture, mix until clumping, then knead until homogeneous.

For the lemon cake, mix the sugar and lemon zest in a large bowl, then add the softened butter, oil, and vanilla, and mix until pale and fluffy. Add the eggs one at a time, mixing well after each.

In a separate bowl, whisk the gluten free flour, almond flour, baking powder, baking soda, xanthan gum, and salt. Add half the dry ingredients to the butter-sugar mixture, whisk to combine, then add the yoghurt and whisk until smooth. Finally, add the remaining dry ingredients and whisk until smooth.

Transfer the lemon cake batter into the lined baking pan and smooth it into an even layer. Spoon the cooled strawberry filling over the batter, ensuring an even layer with some batter peeking through.

Crumble the streusel topping over the strawberry layer evenly. Bake at 350ºF (180ºC) for 40-45 minutes until the topping is golden brown and a toothpick comes out clean.

Allow the cake to cool until warm before removing from the pan, then serve warm or cooled completely. Optionally, dust with powdered sugar before serving.