In order to make this omelette fluffy, whisk egg yolks and whites separately, then carefully fold them together.
- Beat eggs with water and season with salt and pepper.
- Heat enough butter in a small frying pan to form a film over the base.
- Pour eggs into hot pan and cook until almost set on top.
- Scatter parmesan cheese and watercress leaves over one half of the omelette, then fold the other half over the top.
- Carefully lift the omelette from the pan and serve at once.