Using the large holes on a grater-shredder, shred the potatoes and apples into a colander. Press out any excess liquid, and then transfer to a bowl.Stir flour and cornstarch. Add fried onion and garlic
On a lightly floured work surface, roll out the dough 3-4 mm thick. Spread potato-apple mixture into a circle in the center of the dough leaving border of dough uncovered along the edge.
Fold the edge of the dough up and over the mixture, leaving the galette uncovered in the center. Place chestnuts on the top of the mixture.