Preheat the oven to 200°C.

Divide the cauliflower heads into florets and quickly blanch in a saucepan of rapidly boiling water. Drain, arrange in an ovenproof dish and set aside.

To make the cheese sauce, melt the butter in a saucepan over a medium to low heat. Once melted, stir in the flour to form a paste or roux.

Cook, stirring, for 2 minutes, taking care not to allow the mixture to darken.

Add the milk, a little at a time, stirring to combine after every addition.

When the mixture begins to boil, reduce the heat and simmer for 10 to 15 minutes, stirring occasionally.

Remove from the heat, stir in the cheese and season to taste.

Pour the cheese sauce over the cauliflower, top with the Parma ham slices and sprinkle with nutmeg and more cheese, if desired.

Bake for 10 minutes or until golden and bubbling.