Mix together the dry ingredients in a large bowl.
Add the butter and, using your fingers cut in the butter until it resembles coarse meal.
Add one Tbsp of water at a time, and mix gently until the dough just comes together.
Divide in half, flatten into two discs, wrap in clingfilm and refrigerate for at least one hour.
Finely mince the sage and mint and infuse them in the warm melted butter. Add the salt.
Chop the mushrooms and thinly slice the apple and the onion.
Preheat the oven at 200°C, static.
Roll out the crust dough into a large circle on a lightly floured surface, about 3-mm thick. Cut out a 22-cm-large circle.
Re-roll the scrapes to make the second disc of dough. Transfer your discs onto a lined baking tray.
Fill each disc with apples, mushrooms and onion, making sure you leave a 2-cm-large border. Drizzle the filling with the infused butter.
Fold the borders of the gallettes and lightly brush with a beaten egg.
Bake for 30 minutes or until your gallettes are evenly golden. Enjoy while still warm.