Preheat the oven to 180° C. Prepare a baking sheet and baking paper.
Roll out the puff pastry. Peel the rhubarb and chop up into cubes.
In one pot heat butter, frozen fruits, brown sugar, and peeled cubes of rhubarb, lemon zest, and lemon juice. Cook until boiling.
Once the fruit mixture starts to bubble, take it off the heat. Add cornstarch and mix well until the mixture is thick enough.
Add mixture right into the middle of the puff pastry. Leave a few centimeters from the sides. Fold the sides in so the galette has the shape of a circle.
Beat the egg and use egg wash to brush the sides. Sprinkle with brown sugar and bake for 20-25 minutes.
Take the galette out of the oven and let cool. You can serve galette warm or cold, but I recommend serving it with a scoop of vanilla ice cream.