Prepare pie crust dough and chill for at least 2 hours before rolling out. (*See notes for using this pie crust)
If making the frangipane, prepare it before rolling out the pie dough.
In a large bowl, add the berries, granulated sugar, lemon juice, cornstarch, cinnamon, nutmeg and pinch of salt. Gently combine until all the berries are coated in the mixture. Set aside.
On a lightly floured work surface, roll out pie dough to about a 16-inch circle. Flour the top of the dough, fold it in half. Flour it and fold it in half again. Your dough should be in a triangle shape. Using a dough scraper, lift the dough onto a piece of parchment paper.
Unfold the dough and center it on the parchment. Using a knife or dough scraper, trim the dough to create a 5-sided shape (a pentagon). See recipe post for picture. Save the dough scraps and set aside (see notes for using these). Cut 1-inch slits in the center of the edge of each of the 5 straight sides. Slide parchment and dough onto a rimmed baking sheet.
Spread the frangipane in the center of the dough, generously using about 1 cup of it. Leave a 2-inch border unfilled. Drain the mixed berry filling from any juices and add them on top of the frangipane, again leaving a 2-inch border around the edge of the dough. Fold the edges of the dough towards the center. Fold in the dough at the slits, as necessary, to create a center star shape. Place the baking sheet with the galette into the refrigerator for 30 minutes while the oven preheats.
Arrange the oven rack on the slot just below the center position. Preheat oven to 400°F/204°C.
Before baking, brush the pie crust with the egg wash. Top with granular sugar for more sweetness and extra crunch.
Bake galette for 40-50 minutes. The berry filling should be bubbling, and the crust should be a golden brown. If the crust is browning too quickly, lightly place a sheet of aluminum foil over the top of the galette. Allow it to cool for 10-15 minutes.
Serve with whipped cream or vanilla ice cream. Enjoy!
Cover and store any leftovers in the refrigerator for up to 4 days.