Preheat your oven to 350°F. While the oven heats, prepare all your ingredients: squeeze the thawed spinach firmly in a clean kitchen towel or cheesecloth until completely dry—this prevents a watery quiche. Mince the garlic clove and slice the mushrooms into 1/4-inch pieces. Having everything prepped and ready will make the cooking process smooth and efficient.

Heat 1/2 tablespoon of oil in a skillet over medium-high heat. Add the sliced mushrooms and cook for 4-5 minutes, stirring occasionally, until they release their moisture and the liquid evaporates completely. This concentrates their flavor and prevents the quiche from becoming soggy. Add the minced garlic and salt in the final minute, stirring constantly until fragrant. Transfer the cooked mushroom mixture to a plate.

Lightly brush the remaining 1/2 tablespoon of oil around the bottom and sides of a 9-inch pie plate. Layer the ingredients strategically: spread the sautéed mushroom mixture from Step 2 on the bottom first, then distribute the squeezed spinach evenly over the mushrooms, and finally crumble the feta cheese across the top. This layering ensures every bite has balanced flavors and textures.

In a medium bowl, whisk together the room-temperature eggs, milk, grated Parmesan, and black pepper until well combined and smooth. I find that room-temperature eggs incorporate more smoothly and create a more cohesive custard. Don't overmix—just stir until the ingredients are fully incorporated.

Pour the egg custard mixture from Step 4 evenly over the layered vegetables and cheese in the pie plate. Top with the shredded mozzarella, distributing it evenly so the quiche bakes with a golden crust. Place the pie plate on the center oven rack and bake for 50 minutes, or until the internal temperature reaches 160°F when measured with an instant-read thermometer in the center. The quiche should be set but still slightly soft in the very middle—it will firm up as it cools.

Remove the quiche from the oven and let it rest at room temperature for 5-10 minutes before slicing. This cooling time helps the custard set properly and makes slicing much cleaner. Slice into wedges and serve warm or at room temperature.