Preheat the oven to 385 degrees Fahrenheit.

Combine the flour, salt, and sugar in a large bowl for the pie crust.

Add the cold cubed butter to the flour mixture, and work it between your fingers until the dough resembles coarse crumbs and there are no large pieces left (the dough should just hold together when pressed in a handful). Alternatively, use the pulse function on a food processor, taking care not to overwork the dough.

Turn the dough out onto a clean floured surface, and add the cold water and vinegar.

Comb through the dough with your fingers until well-combined, then begin to knead the dough until it comes together, no more than a couple of minutes.

Divide and form into a shaggy disc about 1-inch thick. Wrap in plastic wrap, and place in the refrigerator for at least two hours.

While the dough rests, melt the butter in a large skillet over medium heat.

Add the mushrooms and ½ teaspoon kosher salt and cook, occasionally stirring, until softened and golden brown in spots, 5–10 minutes. Season with more salt, to taste.

Transfer the mushrooms to a medium bowl and stir in scallions, garlic, sugar, and thyme.

Whisk the goat cheese with 1 tablespoon water in a separate bowl until light and fluffy.

Whisk in the lemon zest, lemon juice, black pepper, and ¼ teaspoon kosher salt. Add more salt and pepper as desired.

Generously dust a clean work surface with flour, and roll the dough into a 12-inch circle.

Gently transfer the dough to a parchment-lined baking sheet.

Working quickly, spread the goat cheese mixture over the center of the dough, leaving a 2-inch border; spread mushroom mixture evenly over the top. Sprinkle with flaky sea salt.

Then, using the parchment paper to guide you, fold the edges of the dough up and onto itself, one section at a time.

Brush the exposed dough with egg wash.

Transfer the sheet pan to the oven and bake galette for 40-50 minutes, until crust is golden-brown. If bits of mushroom begin to burn in the high heat before the crust is finished, simply tent a piece of foil over the filling and continue baking.

Cool the galette to room temperature before serving. Finish with a few cracks of black pepper, garnish with chives, and serve with sour cream, if desired.

Leftover mushroom galette can be refrigerated or frozen. Reheat in the oven at 350 degrees Fahrenheit for 15 minutes or until warmed through.