1. Preheat the oven to 210°C (410°F).
2. Cut the cauliflower into small florets and remove the core.
3. Cook it in boiling salted water for 20 minutes, then drain.
4. Heat the milk in a saucepan and set it aside.
5. In another saucepan, heat the oil and simmer the tomato puree with the flour, salt, and pepper, stirring constantly.
6. Gradually pour in the hot milk while stirring continuously, then let it thicken for 3 minutes.
7. Remove the saucepan from the heat and add the grated cheese while continuing to mix.
8. Place the cauliflower in a lightly buttered gratin dish, pour the sauce over it, and bake for 10 minutes.