Cook the bacon in a cast iron skillet over medium-high heat until cooked through. Remove the bacon from the pan and reserve the bacon grease, setting the pan off to the side for now. Crumble the bacon, once cooled, and set aside.
In a medium to large bowl, whisk together eggs, egg whites, and milk. Set aside.
In the bacon grease, sauté the onions for 2-3 minutes until tender. Add tomato and spinach and cook for 1-2 more minutes. Spinach should just begin to wilt.
Pour your egg mixture in the skillet and add bacon pieces. Let cook for 4-6 minutes. The eggs should just begin to set.
Bake for 8-12 minutes, until eggs are completely cooked through.