Preheat oven to 375 degrees F. To a medium skillet, add butter and heat over MED/MED-HIGH heat. Add onions and ham, saute for 5 minutes, stirring occasionally, until edges are deeply caramelized and golden. Remove from heat and set aside.
Cook bacon, then crumble and set aside.
Place open box of frozen spinach on a microwave safe plate, pour 2 tsp of water on frozen spinach, close lid and microwave for 6 minutes. Transfer defrosted spinach to a mesh sieve or colander. Let cool for a few minutes, then press with several paper towels and press to remove excess moisture.
To a medium mixing bowl, add eggs, and half and half and whisk to combine. Add salt, pepper, nutmeg, onions, ham, bacon, and spinach. Stir to combine.
Add chopped mushrooms to same sieve or colander you used for the spinach, and press with paper towels to remove excess moisture. Stir mushrooms and 1 cup shredded cheese into the egg mixture.
Remove frozen pie crust from freezer and transfer egg mixture to pie crust. It will be very full, and depending on your pie crust, all the mixture may not fit. Good news is, the mixture can be used to make a fabulous egg scramble!
Sprinkle top with remaining 1/3 cup shredded cheese. Bake quiche for 40-45 minutes, until top is golden brown and center is set. Remove from oven and let rest for 5 minutes or so. Cut into 8 wedges and serve. Sprinkle with parsley if desired.