Preheat your oven to 400°F and position the oven rack in the middle.
Thaw the pie crust until soft, then prick the bottom and sides with a fork. Bake for 10 to 15 minutes or until lightly golden.
While the crust bakes, melt butter in a skillet over medium heat. Add the onion and cook until soft, about 3 minutes. Stir in the spinach and cook until wilted.
In a bowl, whisk together heavy cream, eggs, Gruyère, Parmesan, salt, and pepper until smooth and creamy.
Spread the spinach mixture evenly on the prepared crust. Pour the custard mix over it, making sure everything's covered nicely.