Bring the slightly salted water to a boil, add the cauliflower, bring to the boil again and cook over low heat until soft. Pour the cauliflower water through a sieve and make up to 750 ml.
Melt the fat over high heat and heat the flour in it, stirring, until it is light yellow. Gradually add the cauliflower water and milk, bring to the boil. Let it steep for 10 minutes over low heat. Stir in the egg yolks, add the cauliflower, season with salt and nutmeg.