Place the flour, salt, granulated sugar and diced cold butter in a food processor. Pulse a few times, until the mixture looks like a coarse meal. Add the cold water while pulsing again a few times until the dough starts to come together and becomes more moist. Turn out of the food processor and gather into a ball. Lightly dust your countertop with flour, and knead the dough 3 to 4 times until it comes together. Shape into a large, flat disk, wrap in plastic wrap and refrigerate until firm, about an hour.
To make the galette:
Preheat the oven to 400°F. Line a rimmed baking sheet with parchment paper and remove the dough from the refrigerator.
In a large bowl, add the grapes, vanilla, granulated sugar, lemon zest and salt. In a small bowl mix the lemon juice and cornstarch together, then add it to the grape mixture. Stir to combine and set aside.
Sprinkle your work surface and rolling pin with flour and roll the dough out into about a 14-inch circle.
Carefully transfer the dough to the lined baking sheet. Spread the 1/2 cup of chopped peanuts onto the dough in a single layer, leaving a 2-inch border. Spread the grape mixture on top of the peanut layer. Fold up the sides of the dough to create a slightly overlapping, pleated crust of about 2 inches over the fruit. Brush the dough with the egg and sprinkle with the turbinado sugar (pressing the sugar into the crust a little with your hands).
Bake for 45-50 minutes until the crust is golden brown and the filling is bubbling. Remove from the oven and transfer to a cooling rack to cool for 10-15 minutes.
To make the topping:
While the galette is cooling, make the topping. In a large bowl stir the peanut butter and confectioners’ sugar with 1/2 cup of the heavy cream until completely smooth and combined. Pour in the remaining heavy cream and beat with an electric mixer until soft peaks form.
Cut the cooled galette into wedges and top with peanut butter whipped cream. Sprinkle around the 2 tablespoons of chopped peanuts.