Make the pastry first by rubbing the butter into the flour until it resembles thick semolina
Add the salt and yoghurt, press together into a flat disk. Wrap and refrigerate for a couple of hours.
Combine the blueberries, lemon juice, sugar, tapioca flour, sea salt together in a bowl. Set aside.
Preheat the oven to 180C.
Take the pastry out of the fridge and roll out the stiff dough into a big circle (20 cms wide) on parchment paper. If it cracks, seal it shut with your finger.
Dump the blueberry mixture in the centre. Fold up the sides.
Lift the parchment paper with the galette onto a baking tray. Using a pastry brush mix the butter and water and brush the pastry borders with the liquid. Sprinkle the sugar and poppy seeds onto the borders.
Bake for 30 minutes or until light golden. Remove, eat warm with a scoop of vanilla ice cream