Dice the onion and chop the potato (without skin) into small pieces.
In large pan, slightly heat the olive oil, then add and cook (over medium/medium-high heat) the onion and potatoes until the onion is transparent and the potatoes are somewhat softened. Then, add the spinach and tomatoes and cook until most of the spinach has begun to wilt. Make sure to occasionally stir so nothing sticks to the pan or burns.
In a bowl, whisk the eggs and egg whites together.
Add the egg mixture to the pan with the vegetables, cover and cook over low heat for 20-25 minutes.
Notes
* To make this dish paleo, try using a Hannah Sweet Potato (less sweet than others) rather than a regular potato
Recipe by LeanMeanKitchen at https://leanmeankitchen.com/spinach-tomato-and-potato-frittata/