Preheat your oven to 375°F (190°C).
In a sauté pan, heat olive oil over medium heat. Add the chopped onion and sauté until soft and translucent, about 5 minutes.
Add the chopped spinach to the pan and cook until wilted, about 2-3 minutes. Remove from heat and let it cool slightly.
In a mixing bowl, whisk together the eggs, heavy cream, salt, black pepper, and nutmeg until combined.
Add the cooled spinach and onion mixture to the egg mixture, followed by the shredded cheese. Mix well.
Place the pre-made pie crust into the pie dish and pour the spinach and egg mixture into the crust, spreading it evenly.
Bake in the preheated oven for 35-40 minutes, or until the quiche is set and the top is lightly golden.
Remove from the oven and let cool for a few minutes before slicing and serving.