Thaw the spinach and squeeze it out well, if necessary chop it roughly. Then peel and finely dice the onion.
Preheat the oven to 180 degrees (top / bottom heat).
Sauté the onion cubes in the oil, add the spinach and cheese. Heat the mixture briefly, then take it off the stove and season with salt, pepper and nutmeg. Now separate the eggs and pull the egg white into the spinach mixture. Divide the spinach mixture into four small, oven-safe molds and place an egg yolk on each one. Let the spinach eggs set in the oven for about 20 minutes.