In the bowl of your stand mixer fitted with the paddle attachment, on low speed, mix the flour, ground almonds, sugar and salt.
Add in the cubed butter, and blend for a couple of minutes or until just small pieces of butter are visible and the flour sticks together when you hold it in your hand.
In a separate small bowl, whisk the egg yolk with the milk and add it to the flour mixture all at once while mixing on low-speed for about 15-20 seconds, or until the dough starts to come together.
If you want to make two smaller galettes, divide the dough in half and wrap each half with plastic wrap. Let the dough rest in the refrigerator while you prepare the filling and crumble topping.
To Make the Almond Crumble Topping:
In a medium bowl, combine the flour, granulated sugar, marzipan, butter and almond extract. You can use a pastry cutter, but I like to use my fingers to blend everything together until large clumps form. Set aside to use after you have assembled the galettes.
To Make the Blueberry Filling:
In a medium-sized bowl, combine the blueberries with the sugar, crumbled almond paste (or marzipan) flour and salt. Mix well. Set aside.
To Assemble the Galettes:
Take the dough out of the refrigerator and let it sit at room temperature for 5 to 10 minutes.
Place a rack in the centre of your oven and preheat oven to 350ºF. Line a baking sheet with parchment paper.
Lightly flour your surface and roll out your dough into a disk that is approximately a 12" circle and about 1/4 inch thick.
Fold the dough in half and transfer onto the lined baking sheet. Top the dough with the blueberry mixture, then spread it out to within 1.5 inches of the edge.
Gather the dough around the blueberries by overlapping the creases of dough over each other slightly.
Sprinkle the crumble topping above the blueberries. And then sprinkle the sliced almonds on top of the crumble.
Whisk the egg white (remaining from the egg used in the dough) until foamy and brush the edges of the galette with it. Sprinkle the top of the galette crust with the turbinado sugar.
Bake galette for 45 to 50 minutes or until the pastry is golden brown and filling is bubbling.
Let the galette cool for 20 minutes before serving warm, or you can let it cool completely and serve it at room temperature. Serve plain or with vanilla ice cream.