To make the pie crust dough, in a food processor, pulse the flour and the salt for 2 seconds. Add the butter and pulse a few times until the butter is the size of peas. Add the water and vinegar and pulse a few more times. The dough will look like a pile of wet crumbs, it does not need to come to a ball. Remove the dough from the food processor and dump on a large piece of Saran wrap. Pick up the corners and twist tightly to make the dough form a solid ball, then press the dough into a disk. If it's still crumbly, gently press with your palms, but do it quickly, you don't want to melt the butter. Refrigerate for 30 minutes.

Position rack in the bottom third of oven, and preheat oven to 375F.

Line a large baking sheet with parchment paper and set aside.

In a skillet, melt butter over medium-high heat. Add the onion, garlic, vinegar and cook until tender, about 5 minutes. Transfer the onions to a bowl, add the cream cheese, blue cheese, thyme, salt, and pepper and mix well with a fork. Set aside.

On a lightly floured surface, roll out the pie crust dough into a 14-inch circle. Spread the onion/cheese mixture on top of the dough, leaving a 2-inch border.

Arrange the vegetables slices in a circle over the onion/cheese mixture, alternating between different vegetables and colors. Sprinkle blue cheese and salt over vegetables. Fold excess dough up and over the vegetables.

Brush the dough with the egg wash.

Bake until the vegetables are tender and the crust is golden brown, 45 to 50 minutes.

Meanwhile, whisk together olive oil, vinegar, and chopped thyme.

Remove the galette from oven, drizzle the oil/vinegar/thyme mixture over vegetables, slice and serve.