First, get your oven ready. Preheating is crucial for an even bake. Set it to 375°F (190°C). While it heats up, place your pie crust into a 9-inch pie dish. If needed, trim the edges for a neat look. Pre-bake the crust for about 7 minutes. This step, known as blind baking, helps prevent the bottom from becoming soggy once you add the wet filling.
Next, heat the olive oil in a skillet or pan over medium heat. Add the diced onion and minced garlic, cooking until they become fragrant and softened. Then, add the sliced mushrooms and chopped spinach. Continue to cook for another 3-4 minutes until the mushrooms have released their liquid and the spinach has wilted. Finally, stir in the lightly steamed broccoli and cook for two more minutes before removing the pan from the heat.
In a large mixing bowl, crack the four large eggs and whisk them until the yolks and whites are well combined. Pour in the milk (or half-and-half) and continue to whisk. Season the mixture with salt, pepper, and a pinch of nutmeg if you're using it. Whisk everything together until the custard is smooth and uniform. This will be the rich and creamy heart of your quiche.
Now it’s time to bring it all together. Spread the sautéed vegetable mixture evenly across the bottom of your pre-baked pie crust. Sprinkle the shredded cheese over the vegetables. Carefully pour the egg custard mixture over everything, making sure it fills the crust evenly. Place the quiche in the preheated oven and bake for 35-40 minutes. You'll know it's done when the center is set (it should only have a slight jiggle) and the top is a beautiful golden brown.
Once baked, remove the quiche from the oven and let it rest on a cooling rack for at least 10 minutes. This is an important step, as it allows the custard to fully set, making it much easier to slice cleanly. If you try to cut it while it’s piping hot, it may fall apart. After it has rested, you can slice it into wedges and serve it warm.