Saute the onions in a medium pan until soft and translucent.
Add the spinach to the onions and cook for a couple minutes more, until the spinach has wilted.
Prepare the egg and milk custard in a separate bowl.
Add the spinach and onion mixture, along with the cheeses to the prepared pie crust.
Pour the egg mixture over the spinach, onion and cheese.
Bake the quiche for 40-45 minutes at 375 degrees. Allow to rest for 10 minutes before serving.
Bake the quiche in a pie pan, set atop a rimmed baking sheet to catch any spills.