Preheat oven to 350 degrees. Butterfly flank steak and pound thin. Season to taste with salt and pepper.
Cook bacon in nonstick skillet over medium heat until crisp. Remove bacon. Add onion to drippings in skillet. Cook 6 minutes or until tender. Add 3 cloves chopped garlic and cook 1 minute. Add spinach and cook 1 minute. Stir in bread crumbs and Parmesan cheese.
To make this Keto friendly, replace the bread crumbs with Parmesan cheese, so that would be 1/2 cup Parmesan cheese in total.