Preheat the oven to 375. Start by cooking the bacon until it is fully done, however you usually do that. Set aside and drain the grease, but leave about a tbsp for cooking the vegetables in.
Chop the onion and mushroom and bring your cast iron skillet to medium heat. Saute the onion until soft, and add in the mushrooms. Cook for 2 – 3 minutes, until also soft, and then add the spinach. Continue cooking on medium, stirring often, until spinach is wilted. turn off the stove top.
Cut / crumble the bacon back into the pan and mix it up with the vegetables. In a bowl, whisk the eggs and spices until fully combined. Pour into the skillet and mix with the fillings.
Place in the oven and bake for 10 – 15 minutes (time will vary depending on pan depth etc). Bake until the top is firm when you press on the middle of it.