This recipe combines instructions for a buttery crust (adapted from New York Times Cooking) with a classic vegetable quiche recipe. The Gruyere cheese provides such a rich flavor that I don't recommend substituting with anything else.
Place flour in food processor. If using unsalted butter, add 1/4 teaspoon of kosher salt and blend together. Cut butter in 1/4 inch chunks and pulse in food processor a few times. Add water one tablespoon at a time and pulse until the mixture starts to come together. NOTE: this won't come together the same way as dough in a mixer will; it will have to be combined by hand in the next step.
Turn out contents of food processor on a lightly floured surface. Combine the wet and dry components until a dough is formed. Knead to ensure that all the dry parts are absorbed. Roll into a ball, then flatten out. Wrap in plastic wrap and place in refrigerator to rest for at least 15 minutes.
While the dough is resting, clean out food processor, then combine half-and-half with beaten eggs, salt, and pepper. Blend thoroughly. Chop spinach, slice mushrooms, and grate cheese and set aside for later.
Preheat oven to 375 degrees F.
Remove dough from refrigerator and place on lightly floured wax paper or a Silpat. Roll the dough out using a rolling pin until it's larger than the diameter of the pie pan you're using.
Lift sheet of wax paper or Silpat with the dough, and flip over on top of pie pan. If using a ceramic pan with a ridge on it, make sure the dough is stretched out to cover the ridge. Flatten out dough to make sure there are no air pockets. Try to make the dough as even in thickness as possible.
Layer spianch, mushrooms, and cheese on the bottom of the pie crust. Pour egg mixture into the crust evenly, making sure to push any cheese down below the surface. If using a ceramic pan with a ridge on it, wrap with aluminum foil or use crust protectors to avoid burining.
Bake for 45 minutes or until the egg is set. Cut into 8 even slices.