Combine the water, yeast, and sugar in a measuring pitcher. Whisk until dissolved and set aside for 5 to 10 minutes until foamy. Whisk in the Chardonnay grapeseed oil.
In the bowl of a stand mixer, whisk together flour and kosher salt (along with grapeseed flour if using). Using the dough hook, beat on low speed, gradually adding the yeast mixture. When the dough begins to come together, add 1 cup of the grapes. Increase the speed to medium-low and mix until smooth, about 2 minutes. Remove the dough from the bowl, form into a ball, and place in a large oiled bowl. Brush the top with oil and cover with plastic wrap or a tea towel. Allow the dough to rise 1 1/2 to 2 hours in a warm place or until doubled in size.
Generously brush a 13-by-9-inch pan or 12-by-9-inch rimmed (quarter) sheet pan with plain grapeseed oil. Gently punch down the risen dough and remove from the bowl. Press the dough into the prepared pan, making sure to stretch the dough into the corners of the pan. Sprinkle with the walnuts and remaining grapes, pushing them slightly in. Brush the top with the remaining plain grapeseed oil and cover with a tea towel. Allow to rise 30 minutes more.
While the dough is rising, preheat the oven to 425°F. Sprinkle the dough with the flake salt and bake for 20 minutes on the center rack. Lower the heat to 350 degrees F and bake 10 minutes more or until golden brown. Cool 10 minutes before cutting and serving.