1. Preheat your oven to 170°C (350°F).
2. Peel the sweet potatoes and garlic.
3. Thoroughly wash the skin of the zucchinis.
4. Using a mandoline, slice the sweet potatoes, onions, and zucchinis, then finely chop the garlic.
5. In a sauté pan, sauté the onions and zucchinis for about 5 minutes with salt, pepper, nutmeg, and garlic.
6. Take a gratin dish, layer half of the sweet potato slices, cover with the sautéed zucchinis and onions, and finish with the remaining sweet potato slices.
7. Pour the heavy cream over the top.
8. Finish by sprinkling grated gruyère cheese on top.
9. Bake in the oven for about 1 hour and 30 minutes, checking the cooking occasionally.
10. Enjoy your meal!