Preheat your oven to 375°F (190°C) and prepare your pie crust in a pie dish. If using a store-bought crust, simply follow the package instructions for pre-baking or use a fork to prick holes in the crust and bake for about 10 minutes to prevent sogginess.
In a skillet over medium heat, sauté the diced onion and minced garlic until fragrant and translucent, about 3-4 minutes.
Add the chopped spinach to the skillet and cook until wilted, another 2-3 minutes. Remove from heat and set aside.
In a mixing bowl, whisk together the eggs, milk, salt, pepper, and nutmeg until well combined.
Spread the sautéed spinach mixture evenly over the pre-baked pie crust. Sprinkle the crumbled feta cheese and grated Parmesan cheese on top.
Carefully pour the egg mixture over the spinach and cheese, ensuring it spreads evenly.
Bake in the preheated oven for 35-40 minutes, or until the quiche is set and the top is golden brown.
Once done, allow the quiche to cool for a few minutes before slicing and serving. Enjoy the delightful aroma filling your kitchen!