Preheat the oven to 375 degrees. Line a baking sheet with parchment paper.
Place the nectarine slices into a bowl then sprinkle them with the lemon juice then gently toss to coat with the juice. Add the sugar and cornstarch to the nectarines then gently toss to coat evenly.
Place the nectarine slices in a spiral inside the pie crust starting one inch from the edge. Sprinkle the top with the raspberries. Fold the edge of the dough over the fruit. Brush the dough with the beaten egg. Sprinkle the edges with a bit of turbinado sugar. Add the diced butter to the top of the fruit.
Place into the oven and bake for 35-40 minutes, or until golden brown. Remove the galette from the oven and let cool for 15-20 minutes before slicing.
Sprinkle the top with powdered sugar and serve with vanilla ice cream or whipped cream, if desired. Enjoy.