Place the courgettes in a colander, sprinkle with salt and leave to drain for 20-30 minutes. Dry them thoroughly with absorbent kitchen paper
Put the shallot, butter and oil in a saucepan and cook gently until the shallot is translucent - but do not let it get brown or even gold. Add the courgette and saute for 2-3 minutes, stirring all the time
Pour in the stock, cover the pan and simmer for 15 minutes
Gently beat together the egg yolks, parmesan, nutmeg, cream and seasoning. Pour the mixture into the soup, stirring well. Boil for 1 minute, then serve at once