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