Chop the onion, tomatoes, and spinach if not already prepared.
Heat olive oil in a small non-stick pan over medium heat.
Add the minced onion and sauté for 2–3 minutes, stirring occasionally until softened.
Add tomatoes, spinach, salt, and pepper. Cook for another 1–2 minutes until the spinach wilts.
Pour in the egg whites evenly over the vegetables.
Cook the Egg White Omelette undisturbed for 2–3 minutes until it begins to set.
Sprinkle parmesan cheese on top.
Gently fold the omelette in half and cook for another minute.
Remove from heat, season to taste, and serve warm.