Prepare the pie dough:

1. Combine flour and ½ teaspoon salt in a mixing bowl, then add cubed butter. Use a pastry cutter (or 2 knives) to incorporate butter into the flour until large pea-sized pieces remain. Add ice water and mix with your hands until most of the flour is incorporated and the dough is smooth.
2. Wrap the dough tightly in plastic wrap and place in the refrigerator to rest and firm up for 30 minutes.
3. Once the dough has firmed up, place it on a lightly floured surface. Use a rolling pin to roll it out into a circle or oval shape, about ¼-inch thick (The edges will be folded over, so the shape doesn’t have to be perfect).
4. Transfer the dough to a large parchment lined baking sheet. Cover in plastic cling wrap and refrigerate until the filling is ready.

Prepare the filling:

1. Preheat oven to 375F.
2. Melt butter in a large skillet over medium-high heat until it starts to bubble, about 1 minute. Add mushrooms and cook without turning for 3-5 minutes.
3. Add onion and garlic and season with salt and pepper. Cook until the mushrooms are golden brown and almost all the liquid has evaporated, about 5 minutes. Stir in thyme.
4. In a small mixing bowl, stir together crème fraîche and 1/2 cup gruyere cheese and set aside.

Assemble and bake:

1. Take the dough out of the fridge and spread the crème fraîche mixture onto the base of the rolled out dough, making sure to leave a clear 1-inch border around the edge.
2. Spoon the mushroom filling on top of the crème fraiche.
3. Top with the remaining 1/2 cup gruyere cheese.
4. Carefully fold over the outer edge over the filling, overlapping about every inch. Brush egg wash along the folded over edges.
5. Bake for 50-55 minutes, or until the dough crust turns golden brown. Let cool for 5 minutes.
6. Let cool slightly for about 5 minutes and garnish with fresh thyme. Slice and serve warm.