Preheat your oven to 375 degrees F (190 degrees C).
In a medium skillet, melt the butter over medium to medium-high heat. Add the diced onions and ham, and sauté for about 5 minutes, stirring occasionally, until the edges are beautifully caramelized and golden. Set aside.
Prepare the bacon by cooking it until crispy. Once cooked, crumble it and set aside.
Place the frozen spinach in an open box on a microwave-safe plate. Pour 2 teaspoons of water over the frozen spinach, close the lid, and microwave for 6 minutes. After defrosting, transfer the spinach to a mesh sieve or colander. Let it cool for a few minutes, then use paper towels to press and remove any excess moisture.
In a medium mixing bowl, whisk together the eggs and half and half until well combined. Add the kosher salt, black pepper, ground nutmeg, caramelized onions, diced ham, crumbled bacon, and drained spinach. Mix everything together.
Press the chopped canned mushrooms in the same sieve or colander used for the spinach to remove excess moisture. Add the mushrooms and 1 cup of shredded Gruyere cheese to the egg mixture. Stir until all ingredients are evenly distributed.
Take the frozen pie crust out of the freezer and pour the egg mixture into it. Be prepared; it will be quite full! If there's any excess filling, you can save it to make an extra delicious egg scramble later.
Sprinkle the top of the quiche with the remaining 1/3 cup of shredded Gruyere cheese.
Bake the quiche in the preheated oven for 40-45 minutes, or until the top turns a glorious golden brown, and the center is fully set.
Remove the quiche from the oven and let it rest for about 5 minutes before cutting it into 8 delightful wedges. Garnish with minced fresh parsley, if desired.