Peel onion and garlic, dice finely and fry in hot butter. Add frozen spinach and 5 tablespoons of water. Cover and simmer for about 15 minutes.
Preheat oven (electric cooker: 150 °C/circulating air: 125 °C/gas: see manufacturer). Crumble the gorgonzola and stir into the spinach. Season to taste with salt, pepper and nutmeg. Divide into four small casseroles.
Beat 1 egg on each. Bake in a hot oven for about 10 minutes. Season egg with salt and pepper. Serve with baguette or mashed potatoes.