Preheat the oven to 400 degrees. Line a baking sheet with parchment paper.

Unroll a pre-made pie crust and place it on a lightly floured surface. Use a 4-inch round pastry cutter (or trace a knife around a 4-inch bowl). Cut three rounds then re-roll the pastry dough and cut out two more rounds for a total of 6 rounds.

Pour the strawberries and rhubarb into a bowl and sprinkle the white sugar, cornstarch, mandarin juice, and zest. Gently stir the fruit until the sugar and cornstarch are evenly covering them.

Evenly spoon the fruit inside the pie crusts starting one inch from the edge. Fold and pinch the edge of the dough very well over the fruit.

Brush the dough with the beaten egg. Sprinkle the edges with the raw turbinado sugar.

Bake in the oven for 15-20 minutes, or until golden brown.

Remove the galette from the oven and let cool for 10-15 minutes.