Preheat the Oven: Preheat your oven to 350°F (175°C) and lightly grease a 9-inch pie dish or quiche pan with a bit of cooking oil or non-stick spray.

Prepare the Spinach: Thaw the frozen spinach completely. Squeeze out as much excess water as possible using a clean kitchen towel or paper towels, then set aside.

Cook the Mushrooms: Heat 1/2 tablespoon of cooking oil in a skillet over medium heat. Add the sliced mushrooms and cook until they release their moisture and start to brown, about 5-7 minutes. Incorporate the sliced mushrooms into the pan and sauté until they release their moisture and begin to turn golden brown, approximately 5-7 minutes.

Combine the Spinach and Mushrooms: In the pie dish, evenly spread the cooked spinach and sautéed mushrooms as the base layer. Crumble the feta cheese over the top.

Prepare the Egg Mixture: In a mixing bowl, whisk together the eggs, grated Parmesan cheese, black pepper, and milk until well combined.

Assemble the Quiche: Pour the egg mixture over the spinach, mushrooms, and feta in the pie dish. Gently shake the dish to ensure the egg mixture settles evenly around the vegetables. Sprinkle the shredded mozzarella cheese evenly over the top.

Bake the Quiche: Place the pie dish in the preheated oven and bake for 40-45 minutes, or until the quiche is set and a knife inserted in the center comes out clean. The surface should have a delicate golden hue and a slight puff.

Rest and Serve: Let the quiche cool for around 10 minutes before cutting it into slices and serving. Enjoy it warm or at room temperature, perfect for any meal of the day.