1. Peel the potatoes.
2. Wash them and thinly slice them.
3. Wash the zucchini, cut off the ends, and thinly slice them.
4. Do the same with the Mozzarella.
5. Preheat your oven to 180°C (350°F).
6. In an ovenproof dish, layer the potatoes, zucchini, and Mozzarella.
7. Season with salt and pepper.
8. Sprinkle a layer of parsley, then repeat the layers until all the ingredients are used.
9. Finish with a layer of Mozzarella.
10. Pour the heavy cream over the gratin.
11. Pour the skim milk and bake for 1 hour.