Make the crust. In a large mixing bowl, whisk together the flour, sugar, salt, chives, thyme and parsley. Using a pastry cutter, or a fork, cut the butter into the flour mixture, until it resembles pea-size crumbles. Slowly add the water, working the dough until the flour and butter are incorporated. Do not over mix. Form the dough into a flat disk shape, wrap in plastic wrap and transfer to the refrigerator for at least 20 minutes before using.

Make the filling. Slice the mushrooms ¼ inch thick. Add the butter to a large skillet set over medium heat. When the butter has melted, add the onions and cook, stirring until beginning to soften, about 3 minutes. Stir in the mushrooms, salt, and pepper, increase heat to medium-high. Cook until the mushrooms are tender and most of the liquid has evaporated, about 5 -7 minutes longer. Turn heat off.

Preheat the oven to 400°F with a rack in the center position. Line a baking sheet with parchment paper.

On a lightly floured surface, roll the dough into a 15-inch circle about ¼-inch thick. Carefully transfer to the prepared baking sheet.

Spread the crème fraîche onto the rolled out crust, leaving a 2-inch border around the edge. Using a slotted spoon, scoop the mushroom mixture out of the pan and place on top of the crème fraîche. Sprinkle the mushrooms with gruyere and thyme. Fold the crust over the filling, pleating as needed.

In a small bowl, whisk the egg with 2 tablespoons of water. Using a pastry brush, brush the egg wash over the folded dough.

Bake in the oven until golden brown and flakey, about 45-50 minutes. If the crust is getting too brown, tent with foil.

Let cool on a wire rack for 5 minutes before slicing and serving.