1-Peel the potatoes and cut into thin: Peel the potatoes and cut into thin slices. Wash the zucchini and also cut into thin slices. Set both aside.
2-Preheat the oven to 180°C (top/bottom heat): Preheat the oven to 180°C (top/bottom heat). Grease a baking dish with butter.
3-In a pot, heat the cream and: In a pot, heat the cream and milk. Add the garlic, some salt, pepper, freshly grated nutmeg, and thyme. Mix well and let simmer lightly so the flavors combine.
4-Spread half of the potato slices evenly: Spread half of the potato slices evenly in the baking dish. Then place a layer of zucchini slices on top. Season with salt and pepper. Layer the remaining potatoes and zucchini on top. Pour the cream sauce evenly over the vegetables so that everything is well covered.
5-Sprinkle the grated Gruyère or Parmesan evenly: Sprinkle the grated Gruyère or Parmesan evenly over the gratin. Cover the baking dish with aluminum foil and bake in the preheated oven for approx. 40 minutes.
6-After 40 minutes, remove the aluminum foil: After 40 minutes, remove the aluminum foil and bake the gratin for a further 15-20 minutes, until the cheese is golden brown and crispy.
7-Let the potato-zucchini gratin cool slightly and: Let the potato-zucchini gratin cool slightly and sprinkle with fresh parsley. Serve as a side dish with meat or fish or as a standalone main course.