This free-form pie is incredibly easy and flexible. Use your creativity and what you have on hand to make it your own - a delicious spring/summer dessert.
Roll out pastry on a lightly floured parchment sheet to 12 inches in diameter. Refrigerate the round (covered, on the parchment) while making the filling.
Mix the apple pieces, rhubarb chunks, vanilla, orange or lemon rind, salt, and crystallized ginger (if using) in a large bowl. Set aside.
Mix the sugars and cornstarch in a small bowl. Add the dry ingredients to the fruit and mix until no dry ingredients show.
Take the pastry out of the refrigerator. (Slide the parchment on a cookie sheet before or after adding the filling.) Spread the graham cracker or cookie crumbs in a circle about 9-inches in diameter. Gently spread the filling over the crumbs, then fold the edges of the pastry toward the middle, overlapping and roughly, leaving a circle of 4-6 inches in the middle.
Add the glaze by brushing the milk over all of the pastry and sprinkling the raw (demerara) sugar over the milk.
Preheat the oven to 400°F/204°C. Refrigerate the galette on the cookie sheet while the oven pre-heats. Bake the galette for 15 minutes, then lower the temperature to 375° F/190° C and continue baking for about 30-40 minutes, until the crust is nicely browned and the filling bubbles. Keep an eye on the galette as it bakes. (See note below about juices from the fruit.) Let the galette cool on the cookie sheet. Serve warm or at room temperature.
The graham cracker or cookie crumbs help absorb the moisture from the fruit. If the juices still run out of the galette during baking, scoop them up with a spoon and cool the juices in a small bowl. Once the galette has cooled down, return the juices to the galette by pouring them over the opening in the center.