Combine the flour, salt and black pepper in the bowl of a stand mixer, stirring briefly until the mixture is aerated. Cut the butter into the mixture until it is pea sized.

Drizzle in the ice water, mixing until the dough comes just together. Shape the dough into a ball, cover with plastic wrap and refrigerate for at least an hour before using.

Flour your surface and using a rolling pin to flatten the dough into a round disk.

In a pan, heat the olive oil over medium high heat. When the oil is hot, add the onion, celery and diced apple. Sauté until the onion is translucent, about 5 minutes. Add the chicken, stir and turn off heat.

Transfer your dough to you baking sheet. Pour the apple/chicken filling in the center of the dough. Top with cheddar cheese chunks and apple slices. Fold the galette edges over the filling, leaving the center open and filling exposed.

Brush the galette crust with the beaten egg. Sprinkle salt and pepper over the top of the galette.

Bake in the oven for 30-40 minutes, or until the crust is golden. Remove from oven and let cool for 10 minutes before serving. Before serving, sprinkle with cinnamon.