1: Wash 800 g potatoes, cut them into pieces and cook in a large pot of salted water until they are tender. I used new potatoes here, if you don't have any, use the classic ones, but peel them.
2: Drain the potatoes, then pour them into a salad bowl. Add 4 tablespoons olive oil and mix well. Preheat your oven to 200°C (390°F).
3: Place a sheet of baking paper in a baking tray. Spread the oiled potatoes on top, and place the cauliflower (here a small one) next to it, which you will also coat with olive oil with a brush. Add salt and pepper.
4: Bake for about 30 minutes, until the vegetables are golden brown. Do not turn off the oven.
5: Butter a casserole dish, and pour the potatoes in the bottom.
10: Finish by 50 g grated cheese and put in the oven...
11: ...for about 25 minutes.
Remarks
If you don't have a spring onion at step 6, replace it with a shallot or remove it. As always with gratins, the proportions are very indicative, so feel free to adapt.