1. Separate the cauliflower into florets.
Peel the potatoes and cut them into 2 or 3 cm cubes.
2. Place the vegetables in the steamer basket of a steamer and steam them,
or put them in a saucepan and cover with enough water to boil.
3. Meanwhile, prepare the béchamel sauce.
4. Dissolve the cornstarch in a portion of the cold milk.
5. Boil the remaining milk in the microwave at maximum power (duration depends on the oven's power).
6. Pour the milk/cornstarch mixture into the boiling milk and stir.
7. Cook for 1 minute at maximum power (cook longer if necessary).
8. Season with salt, pepper, and nutmeg.
9. Add oil if needed.
10. Arrange the vegetables in a gratin dish.
11. Pour the béchamel sauce over the vegetables.
12. Sprinkle with grated cheese.
13. Bake in the oven at 425°F or under the broiler, depending on the available options.