(Skip this step if using a store-bought crust.) In a food processor, pulse together flour, salt, and butter just until pea-sized balls start to form. Add the water and pulse again until dough looks just moistened. Dump equal amounts onto two pieces of cling wrap, form both sections into "brick" shapes, wrap tightly, and chill for two hours, minimum. Roll out one of the dough shapes in between two pieces of parchment paper until your crust is approximately 10 inches in diameter.
Press your crust into the pie dish, trim any excess, and roll the outer edge under to create a thicker border along the crust. Use a fork to poke vent holes into the bottom and sides.
Freeze the crust in the pie dish for 30 minutes. Preheat your oven to 350 degrees.
Right before baking, lay a piece of wax/parchment paper into the crust, and dump your baking weights in. Spread the weights around so they are evenly spaced.
Bake the crust for 20 minutes. Remove the pie weights, and bake again for 10-15 minutes, until the crust is browning.
While the crust is baking, saute your mushrooms, onions, and garlic together until all the moisture is released. Add some cracked pepper for flavor. Let the veggies rest on a paper-towel-covered plate.
Saute your spinach until it is wilted, and add it to the other plate of veggies.
In a bowl, whisk together the eggs, cream, milk, and spices, and set aside.
Once the crust is done baking, cover the bottom with about half of the shredded cheese. Spread the sauteed vegetables on top of the cheese, and then pour the egg mixture over that. Top with the remaining cheese.
Bake for 30-40 minutes. Quiche will be done when egg mixture is set, but "wiggles" in the center; a knife inserted in the center will be mostly clean, aside from some moistness.