Peel the potatoes and wash the zucchini. Cut both into not too thick slices (the zucchini can be a little thicker than the potatoes). Spread in a buttered casserole dish rubbed with garlic. It is best to layer carefully like roof tiles, then the liquid can distribute evenly.
Mix the vegetable stock with the cream and the parmesan. Spread over the vegetables, then season with salt and pepper. Bake in the preheated oven at 210 ° C for about 50 minutes. Cut the mozzarella into slices and after about 30 minutes of baking, spread it over the gratin.
Wash the rocket and drain well or spin dry with the salad spinner. Lightly brown the pine nuts in a non-stick pan without adding any additional fat. Take the gratin out of the oven, spread the rocket and pine nuts on top before serving.