First make the dough. In a food processor, pulse together the flour, sugar and salt. Add in cubed butter, and pulse until butter forms the size of peas.
1 1/2 cups All-Purpose flour, 1 Tbsp. sugar, 1 1/2 tsp. salt, 1 1/2 sticks (12 Tbsp.) cold, unsalted butter, cut into cubes
Slowly stream in the ice cold water, 1 Tbsp.at a time. Pulse until dough comes together.
3-4 Tbsp. ice water
Remove the dough from the food processor and onto a lightly floured clean surface. Gently roll/form dough into a ball, flatten into a thick disk and wrap in plastic wrap. Place in fridge to cool for 30-45 minutes.
Preheat the oven to 400°F.
Make the grape filling. Place the halved grapes in a large bowl with lemon juice, vanilla, sugar and cornstarch. Stir and let rest for 5-10 minutes.
4 cups grapes, halved, 1 lemon juiced and zest, 1 tsp. vanilla, 1/3 cup brown sugar, 2 Tbsp. cornstarch, pinch cinnamon and nutmeg (optional)
Sprinkle a clean work surface with flour. Roll the chilled dough out into a 12 inch circle.
Transfer dough to a baking sheet lined with a Silpat mat or parchment paper.
Using a slotted spoon, arrange the grapes on the dough, leaving the juices behind in the bowl.
Fold the rim of the dough up and over the edge of the filling, overlapping the dough as you go around and pleating the dough.
Before baking, use a pastry brush to brush the egg wash all over the dough. This will give the galette a nice brown color while baking. Sprinkle with turbinado sugar for a nice crystallization. 1 large egg, for egg wash, turbinado sugar (optional)
Bake for 30-40 minutes. Let cool slightly then enjoy as is or with ice cream.