Place the cauliflower florets into a saucepan of well salted boiling water and cook for 4 minutes until just tender, adding the silverbeet at the last 1 minute. Drain thoroughly, then transfer to a baking dish.
Melt Farmer’s Table butter in a medium sized saucepan over medium heat then whisk in the flour until a thick paste forms. Continue to whisk the roux over the heat for 5 minutes until it loses its raw smell but does not colour.
Pour the milk over the roux in a slow steady stream, continuing to whisk the whole time. Turn the heat to low and simmer for 10 minutes until thick. Remove the saucepan from the heat then stir through the grated cheese and cream. Season to taste with freshly grated nutmeg.
Preheat the grill to medium high.
Pour the cheese sauce over the cauliflower and silverbeet then top with the extra grated cheddar and grill until golden and bubbling. Serve gratin on its own or as a side to roasted meats.