Wash green onions, chop the heads off, peel one layer then chop them in small pieces. Roughly chop the spinach and set aside as well. Cut cherry tomatoes in half. If using campari tomatoes, slice in round thin slices.
Crumble the feta cheese with your hands or with a fork. Set aside. Pre-heat oven to 350 degrees Fahrenheit.
Add eggs in a bowl, whisk them. Then add egg whites, milk, salt pepper and whisk again.
Place cast iron skillet on medium heat. Once skillet is hot, add olive oil. Then throw the green onions in. Stir to saute them lightly. Add spinach slowly and let it wilt, stir occasionally to turn it over. Volume will go down fast. Let the liquid coming out from spinach evaporate.
Pour whisked eggs over and add feta cheese all over the skillet, try to spread the cheese evenly. The sides of the frittata will start to set but the middle should be all liquid still. Insert a spatula on the sides to let more of the egg mix to flow underneath without breaking the frittata shape. Add the cherry tomatoes halves on top, spread them around evenly.
After it's cooked for few minutes, transfer cast iron skillet to oven and let cook for another 8-10 minutes until the middle of the skillet is not jiggly anymore.
Get the frittata out of the oven, make sure to not let it overcook. Let it cool for few minutes before cutting into it. Usually I cut ours in 6 slices. Serve with additional sides for breakfast / brunch or light meal:). Enjoy!
Video
Notes
Frittata can be served warm or cold (personal preference).
See video for more details.
Storage - you can store frittata in the refrigerator for the next day on an air tight container. I have never frozen mine so I don't know how well it holds frozen.