Preheat oven to 400°F.
Heat a large, deep, oven-safe skillet over medium heat. Cook bacon until crisp. Remove bacon and set aside.
Wipe out all but 1 tablespoon of bacon grease.
Add onions to the hot skillet and cook for 3 minutes or until they begin to soften.
Add garlic and cook an additional minute.
Stir in mushrooms. Cook until mushrooms begin to soften.
Season vegetables with salt and rosemary.
Add spinach.
In a large bowl, whisk together the eggs, milk, salt and ⅓ of the cheese.
Add ⅓ cup of cheese to the vegetables along with 3 strips of the bacon, chopped.
Pour eggs mixture over the vegetables. Top with bacon.
Bake frittata for 10 minutes or until edges are golden brown and the center has set.
Top frittata with remaining cheese. Broil with for 1 minute or until bubbly.
Serve with extra cheese if desired.