Preheat the oven to 375°F (190°C).
If using a store-bought pie crust, fit it into the 9-inch pie dish and prick the bottom with a fork to prevent bubbling. If making homemade, roll out the dough and place it in the dish.
Place the pie crust in the oven for about 10 minutes until lightly golden. Remove and set aside to cool slightly.
While the crust is baking, heat a frying pan over medium heat. Add the chopped spinach and sauté for 2-3 minutes until wilted. Remove from heat and let cool.
In a mixing bowl, whisk together the eggs, milk, salt, black pepper, and nutmeg until well combined.
Add the ricotta cheese, mozzarella cheese, sautéed spinach, and Parmesan cheese to the egg mixture. Stir until all ingredients are well incorporated.
Pour the spinach and cheese mixture into the pre-baked pie crust, spreading it evenly.
Bake in the oven for 30-35 minutes, or until the quiche is set and the top is golden. A knife inserted in the center should come out clean.
Remove from the oven and let it cool for about 10 minutes before slicing. Serve warm or at room temperature.