For the potato omelette, lightly brown the diced onion in the oil, add the diced potato and fry, covered, over a low heat for about 5 minutes.
Melt the butter, add the whipping cream and cook the spinach in it. Season with salt, pepper and nutmeg. At the end fold in the parmesan.
Whisk the egg with the milk and flour and pour over the potato and let it set. Serve the potato and spinach.