In a mixing bowl, whisk together the egg whites, milk, salt, and black pepper until well combined and slightly frothy.

Heat a non-stick skillet over medium heat and add the olive oil or spray with cooking spray.

Pour the egg white mixture into the skillet and cook for about 2-3 minutes, or until the edges start to set.

Gently lift the edges of the omelette with a spatula, allowing any uncooked egg to flow to the edges.

Once the bottom is set but the top is still slightly runny, add the spinach, tomatoes, and cheese (if using) to one half of the omelette.

Carefully fold the other half over the filling and cook for an additional 1-2 minutes, until the cheese melts and the omelette is cooked through.

Slide the omelette onto a plate and serve immediately.