In a medium frying pan, heat the olive oil. Sauté the onion and garlic in it. Add soup, oregano, pelati and tomato puree and let it bubble. Make the sauce with the lid closed in about twenty minutes. At the end, season strongly with salt and freshly ground pepper.
While the sauce is cooking, cut the cauliflower and romanesco in larger roses from the stem. Cook in not too much salted water or over steam until not too soft. Drain carefully and rinse when cooled.
Spread half of the tomato sauce evenly in a greased baking dish. Place the cauliflower and romanesco roses inside. Spread the remaining tomato sauce evenly between them. Season the cream with salt and pepper and pour over the vegetables. Cut the mozzarella into slices, cut them in half and place over the gratin form. Spread the pine nuts evenly between the cheese.
Bake the gratin in the oven heated to 220 degrees on the second rack from the bottom for twenty to twenty-five minutes until golden. Bring to the table hot.
Serve with boiled potatoes or steamed small new potatoes.