Heat the oven to 200C/390F/gas mark 6 and rub the butter around a one-litre baking dish. Scatter over a tablespoon of the grated cheese and shake around the dish to coat.
Bring a pan of salted water to a boil and blanch the romanesco for four to five minutes, until just tender, then drain and leave to steam dry for five minutes. Once dry, finely chop and transfer to a bowl.
Beat the egg yolks and cream in another bowl until slightly thickened, then stir in the remaining cheese, nutmeg and chopped romanesco, and season well. In a third bowl, whisk the egg whites to stiff peaks.
Stir two tablespoons of the egg whites into the romanesco mix and, once incorporated, carefully fold in the rest, taking care not to over-mix and knock out most of the air.
Tip into the baking dish so the mix comes right up to the rim, dust with a little extra cheese, and bake for 30-35 minutes (but check after 25 or so minutes; you don’t want it to take on too much colour) – when cooked, the sformata should have a slight wobble in the centre and a golden top.