Put the spinach in a sieve and pour over boiling water. Cool and squeeze out moisture.Heat the olive oil in a non-stick pan, add 1/2 pat of butter, the leek and mushrooms, and cook until soft. Remove and keep to one side.Whisk the egg yolks with 1 tbsp of water in a bowl. Whisk the whites until they form soft peaks and fold them into the egg yolks, keeping in as much air as possible. Add the remaining butter to the pan, then add the eggs. Cook for 1 minute or until the base is set, then put the mushroom mix, spinach and cheese on half the omelet. Flip the bare half over. Cook for a few minutes, then serve.