For the Pie Dough:

1. Line a rimmed sheet pan with parchment paper or a silicone mat.
2. Using the paddle attachment on a stand mixer, a food processor, or a pastry cutter, incorporate the butter into the flour and salt until it is broken into pea-sized pieces.
3. Add the ice water and mix until the water is absorbed.
4. Turn onto a clean counter and knead lightly one or two times, to form one smooth disc.
5. Wrap in plastic wrap. Refrigerate for at least one hour.
6. When ready to make the filling, remove the dough from the refrigerator and let sit at room temperature for 10 minutes.
7. Place on a floured work surface. Sprinkle the top of the dough with flour, and roll it out with a rolling pin to about a 12-inch circle. Carefully transfer to the prepared sheet pan.

For the Galette:

1. In a medium-sized bowl, combine the grapes, brown sugar, cornstarch, thyme, and lemon juice with a sturdy spoon. Let sit for 5 minutes.
2. Remove the grape filling from the bowl with a slotted spoon, letting most of the liquid drain out. Pile the filling in the center of the dough circle. Fold the edges over the top of the grapes, pleating it all around the edge.
3. Refrigerate on the sheet pan for 30 minutes to re-chill the dough. Preheat oven to 350°F while the dough is chilling.
4. When ready to bake, prepare the egg wash by whisking together the egg and milk. Brush the crust with the egg wash and sprinkle the crust evenly with the turbinado sugar.
5. Bake for 40 minutes, or until the crust is golden brown and the filling is bubbling.
6. Remove from the oven and let rest for 5 minutes on the sheet pan before transferring to a platter and serving warm. Enjoy!