1. Wash the cauliflower, separate it into florets, then peel the potatoes and cut them into slices.
2. In a large pot, bring salted water to a boil and cook the potatoes for 8 minutes. Then add the cauliflower.
3. Cook the vegetables, making sure they don't become too soft, then drain them. Reserve 25 cl of the vegetable broth.
4. Preheat your oven to 240°C (thermostat 8). Prepare the sauce: Melt the butter in a saucepan and stir in the flour. Gradually add the reserved broth, milk, and season with salt, pepper, and nutmeg to taste. Simmer for 10 minutes until the sauce thickens, then adjust the seasoning.
5. Reserve 3 tablespoons of grated gruyère cheese and stir in the rest into the sauce.
6. In a buttered gratin dish, arrange the potatoes and cauliflower. Cover them with the sauce and sprinkle the reserved 3 tablespoons of grated gruyère cheese on top.
7. Bake in a preheated oven and let it gratin for 10 to 15 minutes. Serve immediately.