To make the crust: Whisk together the dry ingredients., Work in the butter until the mixture is crumbly., Toss in the cheese, then drizzle in the water, stirring gently until all is evenly moistened., Pat the dough into a disk, wrap, and refrigerate for 30 minutes., To make the filling: Heat the butter over medium heat in a large pan until foaming., Add the onions and cook for 5 minutes., Add the mushrooms and cook until the mushrooms start to lose their juices, about 10 minutes., Mix in the garlic, thyme, ½ teaspoon of the salt, and the pepper. Stir to combine; remove from the heat., Preheat the oven to 375°F., Combine the eggs, milk, and remaining 3/4 teaspoon of salt., To assemble the quiche: Transfer the dough to a floured surface, and roll it ¼" thick. Line a 9" pie pan with the dough, crimping the edges., Line the bottom of the crust with the cheese, top with the mushrooms, and pour in the egg mixture., Bake the quiche for 35 to 40 minutes, until the edge of the crust is brown, and the filling appears set., Remove the quiche from the oven, and serve warm.