Preheat the oven to 375°F (190°C).
In a large skillet, heat the olive oil over medium heat. Add the diced onion and sauté until translucent, about 5 minutes.
Add the minced garlic and fresh spinach to the skillet. Cook, stirring frequently, until the spinach is wilted, about 3-4 minutes. Remove from heat and let it cool slightly.
In a mixing bowl, whisk together the eggs, milk, oregano, salt, and pepper until well combined.
Stir in the cooled spinach mixture and crumbled feta cheese. If using, add the grated Parmesan cheese.
Pour the filling into the pre-made pie crust, spreading it evenly.
Bake in the preheated oven for about 30-35 minutes, or until the quiche is set and lightly golden on top.
Allow the quiche to cool for 5-10 minutes before slicing and serving.