Start thawing your spinach in a bowl. Set aside.
Preheat your oven to 375 degrees. Follow the directions on the ready made pie crust package to partially bake the crust. You basically want to bake the crust for about 7-8 minutes and then remove from the oven so you can fill it.
Over medium-high heat, add the olive oil and saute the onions and mushrooms for about 2-3 minutes. Add the spinach and stir. Cook for about 2-3 minutes more. Season with salt, pepper and nutmeg; taste carefully for seasoning.
In a mixing bowl, beat the eggs and add the milk or heavy whipping cream. Then add the spinach and mushrooms mixture; stir well. Have this ready when the partially baked pastry shell is coming out of the oven. Pour the mixture into the hot shell. Sprinkle the cheese over the top. Bake at 375 degrees for about 25-30 minutes. The quiche is done when the cheese has been browned a little and an inserted knife comes out clean. Sprinkle with a little more salt on top if desired.