If using a homemade pie crust prepare it at least 3 hours in advance so it has time to chill.
Spray a 9" tart pan or 9" pie pan with non-stick cooking spray. Roll out the chilled pie dough and press it into the pan, use a fork to poke several holes in the bottom of the pie crust. Keep chilled in the refrigerator while you prepare the filling.
Preheat oven to 375°F.
Heat the olive oil in a medium pan and add the chopped onions. Sauté until soft. 3-5 minutes, set aside.
Defrost the frozen spinach in a microwave and use a paper towel to wring out all of the water. Make sure all of the moisture is removed or you will have a watery quiche with a soggy crust.
In a large bowl whick to combine eggs, half & half, salt, and pepper.
Remove the pie crust from the fridge. Add the onions then top with half of the chopped spinach. Then top with half of the shredded cheeses, the rest of the spinach, rest of the cheeses. Pour in the egg mixture and bake for 35-45 minutes until set and a knife comes out clean.
Allow the quiche to cool for a few minutes before slicing and serving.