Preheat the oven to 375 degrees. Line a baking sheet with parchment paper.
Unroll a pre-made pie crust and place on the prepared baking sheet.
Pour the raspberries and blackberries in a bowl and sprinkle the white sugar, corn starch and lemon zest on top. Gently stir the blackberries until the sugar and corn starch are evenly covering the berries.
Pour the berries inside the pie crust starting one inch from the edge. Fold the edge of the dough over the berries. Brush the dough with the beaten egg. Sprinkle the edges with the raw turbinado sugar. Bake in the oven for 35-40 minutes, or until golden brown.
Make the lemon whipped cream while the galette is baking by combining the ice cold heavy cream, powdered sugar, lemon zest and lemon juice together in a bowl.
Using a beater, mix on high until the whipping cream is thick and fluffy. Refrigerate until needed.
Remove the galette from the oven and let cool for 10-15 minutes before slicing. Serve with lemon whipped cream. Enjoy.