In a small bowl, whisk the egg whites until light and fluffy, about 1 minutes.
Heat a large non-stick pan on medium heat. Drizzle pan with olive oil then saute onions for 2 minutes until translucent.
Add sun-dried tomatoes, spinach and saute an additional minute or so until spinach is wilted. Season with a bit of salt and pepper to taste.
Using a rubber spatula, move all of the veggies into the center of the pan. Pour in egg mixture and reduce heat to medium. As the eggs cook, use the spatula to lift up the cooked eggs and hold the pan allowing the runny mixture to move to the outside edge. Continue doing this method until you can easily move your spatula around the entire edge of the pan.
During the last minute of cooking, sprinkle feta on top. Fold over eggs before serving.
To separate yolk from egg whites, carefully pour the yolk back and forth between the two shell halves, allowing the whites to slip out. Alternatively, hold the egg gently in your hand letting the whites slip through your fingers.