Heat a non-stick skillet over medium heat, then add half of the olive oil or a light mist of cooking spray.
Chop the spinach and cut the tomatoes into halves, then add to the skillet. Sauté for 1-2 minutes, or until spinach is wilted.
In a medium bowl, combine the egg whites with sautéed vegetables, salt, and pepper. Lightly beat with a fork or whisk until the mixture becomes slightly foamy. For added flavor, you can mix in other ingredients like cheese or cooked bacon.
Return the skillet to medium heat and add the remaining olive oil to grease the surface. Once heated, pour in the egg white mixture, tilting the pan to spread it evenly.
Cook on low heat for 3-5 minutes or until the egg whites have set. Use a spatula to gently fold the omelette in half.
Season with more salt and pepper to taste, then serve immediately.