Preheat oven to 350 degrees F.

Cook the bacon in a cast iron skillet over medium heat until it reaches desired level of crisp, about 2 to 3 minutes per side. Transfer bacon to a cutting board, leaving bacon drippings in the skillet. Once cool enough to handle, chop bacon and set aside.

Add the butternut squash to the skillet with the bacon drippings and cook over medium heat, covered, until squash begins to soften, about 5 minutes. 

Add the mushrooms and continue cooking until mushrooms soften and begin turning brown, about 5 minutes, stirring occasionally.

Add spinach, oregano, and sea salt. Cover and cook until spinach has wilted, about 2 minutes. Add the chopped bacon. Remove skillet from heat.

Pour the eggs into the cast iron skillet and use a wooden spoon to gently shift mixture around to be sure the vegetables are evenly distributed throughout the frittata. If you're adding cheese to the frittata, now's the time to do so.

Place on center rack of preheated oven and bake 35 to 40 minutes, or until frittata is golden-brown and cooked through. Let sit 10 minutes before serving.