Combine all the crust ingredients except the ice water in the bowl of your food processor, and run until the ingredients start to come together. Add in the ice water until  a smooth dough forms. Form the dough into a ball, wrap in plastic wrap, and transfer to the fridge to rest.

In a skillet over medium heat, saute the onion and celery with some butter or olive oil until translucent. Add the sliced apples and thyme, and cook for 3-4 minutes or until the apples are just slightly softened. Remove from heat.

Prepare a large piece of parchment paper, and dust lightly with flour. (see note below) Remove the crust from the fridge, and on the prepared parchment paper, roll the crust into about 1/2 inch thick circle. Spread the apple mixture into the middle of the crust, leaving an inch overhang at the edges. Fold the edges up over the filling, and then transfer the galette into the fridge to rest for 20 minutes. Meanwhile, preheat the oven to 375 degrees F. 

Sprinkle half of the shredded gruyere over the filling, and then bake the galette on a baking sheet, in the preheated oven for 40-45 minutes or until crust is brown and filling is bubbling. Sprinkle the remaining cheese over the galette immediately, including over the crust so it melts in. Allow to cool, and then slice into wedges and serve!