Peel, wash and cut the potatoes into pieces and cook them in boiling salted water for about 10 minutes. Meanwhile clean and wash the Romanesco and cauliflower and remove the florets from the stems. Cook in boiling salted water for about 5 minutes.
Wash, clean and halve the tomatoes. Drain the potatoes and let them evaporate. Peel onion and chop finely. Heat the fat in a pot and fry the onion. Dust with flour and sweat for about 1 minute.
Stir in stock and milk and bring to the boil while stirring. Let simmer for about 10 minutes. Remove from heat and stir in cheese. Season with salt and cayenne pepper. Put potatoes, romanesco, cauliflower and tomatoes into an ovenproof casserole dish.
Pour the sauce over the top and bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 20 minutes. Remove and serve immediately.