In a large mixing bowl, combine flour, sugar, semolina, sugar and salt. Grate the butter and mix with hands until the mixture resembles coarse meal.
Add the buttermilk slowly and bring everything together to form a ball. Wrap with a plastic wrap and refrigerate for at least 45 minutes.
To make filling
Combine blueberries, sugar and corn flour in a bowl and set aside.
To make the streusel
In a bowl combine flour, brown sugar, almond meal, almond extract and sliced almonds. Grate the butter and mix with hands until well combined.
To assemble galatte
Preheat oven to 180 °C | 350 °F.
Remove the dough from fridge and place on a parchment paper. Roll the dough into a 14-15 inch circle.
Place the blueberries in the center of the dough, leaving a 2-inch border. Sprinkle the almond streusel over the blueberries.
Fold the border over the blueberries, pleating where necessary and pressing gently to adhere the folds. Lightly brush the dough with the egg and sprinkle turbinado sugar.
Transfer to a baking sheet and bake for 45 minutes or until the crust is golden brown. Transfer to a wire rack to cool before slicing.
Notes
You can make the galette dough in advance. Keep for 10 minutes at room temperature before rolling.
For the srreusel topping, almond meal and essence is optional.