While the cauliflower is in the oven you can prepare the feta sauce. Start by melting the butter in a saucepan then whisk in the flour and let it heat up another minute while stirring
To save some time I usually heat up the milk and cream in the microwave before adding it to the pan. Add salt and rosemary, whisk until smooth and bring to a boil, then turn down the heat and let it simmer for 10 min.
Add the feta cheese in crumbles and whisk until melted. Cook until melted then finish with a pinch of nutmeg.
Serve the cauliflower hot and pour over feta cheese sauce!