Melt the butter, add the marjoram, and cook for a minute. Add the spinach, and stir to combine the flavours. Season, leave to cool, then chop roughly.
In a large bowl, lightly beat the ricotta with a fork, then sieve in the flour. Add the egg yolks, the nutmeg and Parmesan and finally fold in the cooled spinach mixture until well combined. Taste for seasoning.
Dust a baking tray