1. Preheat oven to 350°.
2. Heat a heavy-bottomed, oven-safe skillet (such as a cast iron) over medium heat. Cook the bacon, in batches, if necessary to prevent overcrowding, until cooked to your liking. Transfer the cooked bacon to a paper towel-lined plate and set aside.
3. While the bacon is cooking, add the eggs to a large bowl and whisk until well combined. Add 1 cup of the cheese, salt and pepper to taste, and whisk again. Set aside.
4. There should be between 1-2 tablespoons of bacon grease left in the pan after the bacon has been cooked. If there is significantly more, remove some, and if there is not at least a tablespoon, add a little olive oil or butter to the pan. Add the mushrooms to the skillet and cook over medium heat until they just start to soften about 3 minutes. Add the spinach and continue to cook, stirring frequently until the mushrooms are cooked through, the spinach is thawed, and some of the excess moisture has cooked away about 5 more minutes.
5. Cut the bacon into small bits, add to the mushroom-spinach mixture, and stir to combine. Add the eggs and cook for about one minute. Using a heat-proof spatula, firmly scrape along the bottom and sides of the skillet every thirty seconds, moving the formed curds and allowing liquid egg to run into the empty spaces. Continue for about two minutes before using the spatula to evenly distribute the frittata in the skillet, lightly smoothing the top if necessary. Top with the remaining ½ cup of cheese and place in the oven.
6. Bake for 10 minutes, or until the center is just set and the cheese is lightly browned. Remove the frittata from the oven and allow it to rest for 5 minutes before cutting into wedges and serving.

Tips for What to Do with Excess Ingredients: Excess frozen spinach can be added to smoothies, eggs, soups, casseroles or used in place of fresh in many recipes. Leftover cheese can be used to top burgers, sandwiches, cut into cubes for a snack, grated and used to top chili, or added to eggs.