- 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!