Heat a nonstick 8-inch skillet over medium heat for 2-3 minutes.
Meanwhile, in a medium bowl, vigorously whisk the egg whites with salt, pepper, and garlic powder, until frothy.
Lightly spray the skillet with olive oil, then pour the egg whites in and start cooking them, tilting the skillet to spread them evenly.
Scatter the tomatoes and green onions on top.
As soon as the edges start to set, reduce the heat to medium-low.
Gently and repeatedly lift the edges of the omelette and tilt the skillet to allow the still-runny egg whites to slide to the bottom of the pan, where they will cook faster.
When the bottom is cooked and the top is not runny anymore but is still wet, carefully flip the omelette.
After flipping the omelette, sprinkle it with the parmesan.
Fold the omelette and slide it onto a plate.
Serve immediately.
