Step 1: Prepare the Pie Crust (if making from scratch)

1. Combine Dry Ingredients: In a large bowl, whisk together the flour and salt.
2. Cut in Butter: Add the cold, cubed butter to the flour mixture. Using a pastry blender or your fingertips, cut the butter into the flour until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
3. Add Ice Water: Sprinkle 3 tablespoons of ice water over the mixture. Gently mix with a fork until the dough just starts to come together. If it’s too dry, add more ice water, 1 teaspoon at a time, until the dough holds together when pressed. Be careful not to overmix.
4. Chill Dough: Form the dough into a flat disc, wrap it tightly in plastic wrap, and refrigerate for at least 30 minutes (or up to 2 days). This allows the gluten to relax and the fat to firm up, resulting in a tender, flaky crust.
5. Roll Out Dough: On a lightly floured surface, roll out the chilled dough into a 12-inch circle, about ⅛-inch thick.
6. Transfer to Pie Dish: Carefully transfer the dough to a 9-inch deep-dish pie plate. Gently press the dough into the bottom and up the sides of the dish. Trim any excess dough, leaving about a ½-inch overhang. Crimp or flute the edges as desired.
7. Chill Again: Place the pie crust in the freezer for 15-20 minutes or in the refrigerator for at least 30 minutes to firm up. This helps prevent shrinkage during baking.

Step 2: Blind Bake the Crust

Blind baking is crucial for a crisp quiche crust, preventing a soggy bottom.

1. Preheat Oven: Preheat your oven to 375°F (190°C).
2. Prepare for Baking: Line the chilled pie crust with parchment paper, ensuring it covers the bottom and sides. Fill the parchment paper with pie weights, dried beans, or uncooked rice, distributing them evenly.
3. First Bake: Bake for 15-20 minutes, or until the edges of the crust are lightly golden.
4. Remove Weights: Carefully remove the parchment paper and weights. Prick the bottom of the crust a few times with a fork.
5. Second Bake: Return the crust to the oven and bake for another 5-7 minutes, or until the bottom is lightly golden and looks dry.
6. Cool Slightly: Remove the crust from the oven and let it cool slightly on a wire rack while you prepare the filling. Reduce oven temperature to 350°F (175°C).

Step 3: Prepare the Spinach Mushroom Filling

1. Sauté Aromatics and Mushrooms: Heat the olive oil or butter in a large skillet over medium heat. Add the chopped onion and cook until softened and translucent, about 5-7 minutes. Add the minced garlic and sliced mushrooms. Cook, stirring occasionally, until the mushrooms have released their liquid and are nicely browned, about 8-10 minutes. Season lightly with salt and pepper.
2. Wilt Spinach: If using fresh spinach, add it to the skillet in batches, stirring until it wilts down completely. This may take a few minutes. If using frozen spinach (thawed and squeezed very dry), add it to the skillet and cook for 2-3 minutes to heat through and evaporate any remaining moisture.
3. Drain Excess Liquid: It’s very important to remove as much moisture as possible from the cooked vegetables. You can tilt the pan and press the mixture with a spoon to drain off any liquid, or even transfer the mixture to a fine-mesh sieve and press gently. Excess moisture will result in a watery quiche.
4. Assemble the Base: Spread the cooked spinach and mushroom mixture evenly over the bottom of the pre-baked pie crust. Sprinkle the shredded Gruyère cheese (or your cheese of choice) evenly over the vegetables.

Step 4: Prepare the Custard and Bake

1. Whisk Custard: In a medium bowl, whisk together the eggs until well combined. Gradually whisk in the heavy cream and whole milk. Add the salt, pepper, and freshly grated nutmeg (if using). Whisk until everything is smoothly incorporated.
2. Pour Custard: Carefully pour the egg mixture over the vegetables and cheese in the pie crust. Pour slowly to allow the custard to settle without dislodging the filling too much.
3. Bake the Quiche: Place the quiche on a baking sheet (to catch any potential spills) and carefully transfer it to the preheated 350°F (175°C) oven.
4. Check for Doneness: Bake for 35-45 minutes, or until the custard is set and the top is lightly golden brown. The center should be just barely set; it will continue to cook slightly as it cools. It should have a slight jiggle but not be liquid. An instant-read thermometer inserted near the center should register 165-175°F (74-79°C).
5. Cool and Rest: Remove the quiche from the oven and let it cool on a wire rack for at least 15-20 minutes before slicing and serving. This resting period allows the custard to fully set, making it easier to slice neatly.