Preheat oven to 350 degrees F. Lightly grease a 1 1/2 quart baking dish and set aside.
In a large skillet cook bacon until crispy over medium to medium low heat. Drain bacon on paper towel. Drain all but 2 tablespoons of bacon grease and add in the mushrooms and onion. Cook for 5-7 minutes or until lightly browned. Add in garlic and cook for 2 minutes. Remove from heat.
In a large bowl whisk together the eggs and milk. Add in the cooked and crumbled bacon, hash browns, spinach, cheeses and mushroom mixture. Stir to combine. Carefully pour into prepared baking dish. Bake for 45 minutes or until the center is set and no longer liquidy.