Preheat the oven to 350°F. Roll the Quick Puff Pastry into a rough 14-inch round and place it on a baking sheet lined with parchment. Dock (prick it gently all over with the tines of a fork) the middle of the dough and refrigerate it for at least 20 minutes.
Meanwhile, in a large pot over medium heat, combine the rhubarb, cranberries, sugar, zest and vanilla. Cook until the fruit juices start flowing and start to simmer.
Ladle approximately ¼ cup of the juices from the pot into a small bowl and whisk in the cornstarch. Pour the mixture back into the pot and stir it into the fruit until combined. Bring the mixture to a simmer and gently stir in the black raspberries and strawberries. Set the pot aside to cool.
Remove the dough from the refrigerator and spoon the filling into the middle of the round, leaving a border of about 1 inch uncovered all around. Gently fold the edges of the dough over the fruit, leaving the majority of the fruit exposed, creating pleats as you go along.
Using a pastry brush, brush the egg wash over the exposed dough. Sprinkle the entire top with sanding or turbinado sugar. As an insurance policy, I’ll place a cake ring the same size as the galette around the sides to provide extra stability and keep everything tidy. Bake the galette for 35 to 40 minutes, until the crust is golden brown.
Adapted from Pie It Forward