In a small bowl, whisk together the eggs, egg white, water, a pinch of salt, and pepper. Keep whisking until the mixture looks pale yellow and slightly foamy. This step only takes about thirty seconds but really affects the final texture.
Place a medium nonstick skillet over medium heat and add the olive oil. Let the oil warm for about a minute before adding anything else. A properly heated pan keeps the onions from steaming instead of browning.
Add the chopped onions to the skillet and cook for three to four minutes, stirring occasionally. You're looking for them to turn tender and slightly golden at the edges. This builds the flavor base for the whole dish.
Stir in the sliced mushrooms and cook for another three to four minutes. They'll release some liquid at first, then start to brown as that liquid cooks off. Stir occasionally so they cook evenly on all sides.
Add the fresh spinach to the skillet and stir constantly. It wilts down fast, usually within a minute. Once it's fully softened and bright green, you're ready for the eggs.
Pour the whisked egg mixture into the skillet with the vegetables. Stir gently and continuously until the eggs thicken and no liquid remains. If you're adding cheese, sprinkle it in during this step so it melts right into the eggs.
Once the eggs look set but still soft, take the skillet off the heat right away. Eggs keep cooking from residual heat, so pulling them early keeps them from turning rubbery. Plate and enjoy while hot.