Cut cold butter into cubes. In the mixing bowl of a stand mixer with a paddle attachment, mix butter, powdered sugar, and flour.
Then add egg yolk, cold water, and lemon zest. Mix again on low speed. The dough should look like wet sand.
Shape the dough into a ball and wrap into plastic wrap. Refrigerate the dough for at least an hour.
How to Make Cream Cheese Filling
In the mixing bowl, add cream cheese, cornstarch, sour cream, and sugar. Using a fork, mix until all is combined.
In a cup or small bowl, using a fork, whip the egg. Add half of that egg mixture into the cream cheese filling. The other half of the egg mixture is set aside.
Add the lemon zest and lemon juice to the filling, then mix again.
How to Assemble and Bake Blueberry Galette
Preheat oven to 375oF (190oC).
Remove the dough from the refrigerator and remove the plastic wrap.
Place the dough on the parchment paper. Starting at the center of the dough, roll it out with a rolling pin into a circle.
Place the parchment paper with the dough on the baking sheet. Add the cream cheese filling into the center of the dough, leaving a 2-inch border.
Then add blueberries on top of the filling.
Gently fold the edges of the dough over the blueberries, overlapping the dough as necessary. Press gently to seal the edges.
Using a pastry brush or fork, brush the edges of the galette with the remaining egg mix. You can also sprinkle with the white granulated sugar.
Bake the galette at 375°F (190 °C) for 30-40 minutes or until the dough is golden and crisp.
Remove galette from the oven and allow to cool on the baking sheet for about 10-15 minutes before slicing.