Step 1: Prepare the Crust (If Making Homemade)

1. Combine Dry Ingredients: In a large bowl, whisk together the flour and salt.
2. Cut in the 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. The goal is to keep the butter cold and create small pockets that will result in flaky layers.
3. Add Ice Water: Gradually add the ice water, one tablespoon at a time, mixing gently after each addition. Mix until the dough just comes together. Be careful not to overmix, as this can develop the gluten and make the crust tough.
4. Form a Disc and Chill: Form the dough into a disc, wrap it tightly in plastic wrap, and refrigerate for at least 30 minutes. This chilling time is essential as it allows the gluten to relax and the butter to solidify, making the dough easier to roll out and preventing shrinkage during baking.

Step 2: Prepare the Filling

1. Sauté Onions and Mushrooms: Heat the olive oil in a large skillet over medium heat. Add the chopped onion and sauté until softened and translucent, about 5 minutes.
2. Add Mushrooms: Add the sliced mushrooms to the skillet and cook until they release their moisture and become tender and slightly browned, about 8-10 minutes. Don’t overcrowd the pan; if necessary, sauté the mushrooms in batches to ensure they brown properly.
3. Add Spinach: Add the chopped spinach to the skillet and cook until it wilts and reduces in volume, about 2-3 minutes. If using frozen spinach, make sure it’s thawed and squeezed dry before adding it to the skillet.
4. Season and Set Aside: Season the vegetable mixture with salt, pepper, and nutmeg (if using). Remove from heat and set aside to cool slightly.

Step 3: Assemble the Mini Quiches

1. Preheat Oven and Prepare Muffin Tin: Preheat oven to 375°F (190°C). Grease a 12-cup muffin tin generously with cooking spray.
2. Roll Out Dough (If Using Homemade Crust): On a lightly floured surface, roll out the chilled dough to about ⅛-inch thickness. Using a cookie cutter or a knife, cut out 12 circles that are slightly larger than the muffin cups. Alternatively, if using store-bought pie crust, simply unroll it and cut out circles.
3. Press Crust into Muffin Tin: Gently press each dough circle into the muffin cups, shaping it to fit the bottom and sides. If you are skipping the crust, ensure the muffin tin is very well greased.
4. Combine Eggs, Milk, and Cheese: In a medium bowl, whisk together the eggs, milk (or cream), and shredded cheese. Season with salt and pepper.
5. Fill the Crusts: Divide the spinach and mushroom mixture evenly among the prepared muffin cups. Pour the egg mixture over the vegetables, filling each cup about ¾ full. Don’t overfill, as the filling will puff up during baking.

Step 4: Bake the Mini Quiches

1. Bake: Bake in the preheated oven for 20-25 minutes, or until the crust is golden brown (if using crust) and the filling is set and lightly golden on top. A toothpick inserted into the center of a quiche should come out clean when they are done.
2. Cool Slightly: Let the mini quiches cool in the muffin tin for a few minutes before carefully removing them to a wire rack to cool slightly further. This allows them to set up a bit more and makes them easier to remove from the tin.

Step 5: Serve and Enjoy!

These Muffin-Tin Spinach & Mushroom Mini Quiches are delicious served warm, at room temperature, or even cold. They are perfect for brunch, lunch, snacks, or appetizers. See the “How to Serve” section for serving suggestions.