Using a paper towels or a clean kitchen towel, ring out the thawed spinach to get rid of the excess water.
Preheat oven to 350 degrees.
Dice the onion and bacon, and sauté in the olive oil on medium heat. Once the bacon starts crisping and the onions turn translucent, add the potatoes in, and cook about 10 minutes until the potatoes are soft. The rendered bacon fat will help keep the sauté going. Add in the spinach and sauté together with some fresh pepper. Once it’s all combined and spinach is wilted, turn off the heat and cool. Add salt to taste.
Crack the eggs in a large bowl, and whisk in the milk, 1 cup of cheddar, 1/2 cup of Pecorino, fresh herbs, and a dash of salt and pepper. Add in the cooled spinach mixture, and make sure everything is combined.
Pour the eggs into your par-baked pie crust. Using the leftover residual egg from the bowl, brush the sides of the pie crust. Add remaining cheeses.
Bake for 45 minutes, or until the eggs are set. Be mindful of the edges not getting too browned. If they start to, carefully cover the quiche with aluminum foil.
Serve with a sprinkle of Pecorino and fresh parsley. It’s great right out of the oven, and can also be served room temperature!