Toss the frozen mixed berries, ½ cup granulated sugar, the lemon juice, cornstarch and salt in a bowl.
Whisk the cream cheese, egg yolk, the remaining 2 tablespoons granulated sugar and the nutmeg in a separate bowl.
Spread half of the cream cheese mixture over the dough, leaving a 2-inch border.
Top with the blueberries.
Fold the edge of the dough over the filling.
Drizzle the remaining cream cheese mixture over the berries; refrigerate 30 minutes.
Preheat oven to 425 degrees F.
Brush the crust with the beaten egg and sprinkle with coarse sugar.
Bake the galette until the crust is golden, 20 to 25 minutes.
Let cool slightly before slicing.