Heat the oven to 350 degrees. Coat an 8x8 baking dish with cooking spray.
In a large skillet, cook bacon pieces over medium heat, stirring occasionally, until slightly crisp. Remove bacon pieces with a slotted spoon and place on a paper towel lined plate to drain.
Remove all but 1-2 teaspoons of bacon grease from the skillet.
Add the onion to the skillet and cook over medium heat until tender, about 3 minutes.
Add the mushrooms and cook until golden, about 4 minutes.
Add the spinach and minced garlic then cook for 1 minute, stirring constantly.
Remove from the heat and set aside to cool slightly.
In a large bowl, whisk the eggs together with the milk.
Stir in the cooled cooked vegetables, frozen potatoes, and most of the shredded cheese.
Add all but a small handful of bacon to the mixture and stir until well combined.
Season with sea salt and freshly cracked pepper, to taste.
Pour the mixture into the prepared baking dish and top with the remaining cheese.
Place into the oven and bake for 30 minutes so the eggs start to set up.
Carefully add the remaining bacon to the top of the casserole.
Bake for an additional 10-15 minutes, or until the eggs are firm and the top is slightly golden brown.
Let stand for 10 minutes before cutting and serving with green onions on top, if desired. Enjoy.