1. Cut the cauliflower into florets and wash them.
2. Cook them in boiling salted water for 10 minutes, then drain them.
Peel the potatoes, cook them in water for 15 minutes.
3. Drain them, cut them into thick slices.
4. Finely slice the onions.
Dry fry them in a saucepan until they become translucent and start to brown.
5. In a bowl, mix the cornstarch with a little milk.
Pour the remaining milk over the onions, add the diluted cornstarch, and, over low heat, whisk until the sauce thickens.
6. Season with pepper and nutmeg at the end of cooking.
7. Arrange the cauliflower florets and potato slices in a gratin dish.
Cover with the lightened béchamel sauce, sprinkle with grated cheese.
8. Bake in a preheated oven at 240°C (thermostat 8), cook for 10 minutes, finishing if necessary with a few minutes under the broiler.