Preheat oven to 350.

In a food processor, combine butter and flour. Pulse until combined and crumbly.

Add the egg yolk, salt, and water. Pulse until entirely mixed.

Turn the dough out on a floured surface. Shape into a 1/2 inch thick disc, wrap in plastic wrap, and refrigerate for 30 minutes.

Flour a sheet of parchment paper. Place the dough on the parchment and roll it out until it is roughly the same size as your pie pan, plus extra for edges. (It does not have to be in a perfect circle, and it will probably fall apart when you transfer it to the pan, but that's ok, it can easily be pressed back together.)

Grease the pie pan and press the dough into the pan, pressing any holes or tears together, and forming the edges. Cover with parchment paper and 1 cup dry rice (or baking beads if you have them.) Bake at 350 for 20 minutes, then remove from the oven and allow to cool.

While crust is baking, heat 1 tbsp olive oil in a large pan over medium heat. Add mushrooms and sauté 5-6 minutes. Season with salt and pepper.

dd spinach to pan and sauté until wilted. Season with salt and pepper and remove from heat.

Grate the Gruyere. In a large bowl, whisk together eggs and milk. Season with salt and pepper. Add mushrooms, spinach, and cheese to egg mixture. Stir until combined. Pour mixture into quiche crust.

Bake at 350 for 45 minutes or until toothpick inserted in center comes out clean and top is lightly browned and bubbly. (If the edges of your crust start to get too dark midway through baking, you can remove the pan from the oven and cover the edges of the crust in foil for the remainder of the baking time.)