Cut broccoli, cauliflower and romanesco into florets from the stalk. Depending on the size, cut smaller if necessary. Blanch in boiling salted water for about 3 minutes.
Coarsely grate the cheese. Bring cream and milk to the boil, remove from the heat. Add cheese, up to approx. 50 g, while stirring. Season with salt, pepper and nutmeg.
Brush the baking dish with oil. Spread vegetables in it and pour cheese milk over it. Dice bread and grind finely in the universal chopper. Mix the remaining cheese and bread and sprinkle on the vegetables.
Bake in the preheated oven (electric cooker: 200 °C/ circulating air: 175 °C/ gas: see manufacturer) for approx. 20 minutes until the gratin steams hot.