Preheat oven to 350° F
Grease a 12-inch baking dish, set aside
Peel the sweet potatoes, onion and garlic.
Slice all the vegetables using a mandoline to ensure an even thickness
Place the sliced vegetables in the baking dish, in an alternating pattern.
Make only one layer to make sure it cooks evenly
In a medium mixing bowl, stir olive oil, herbs, and garlic, season with salt and pepper.
Drizzle the mixture over the vegetable gratin
Cover the dish with foil and bake for 30 minutes or until the vegetables are al dente
If desired, remove the foil, add a handful of grated cheese and bake for another 5-10 minutes until the cheese is melted and grilled
Insert a toothpick or knife in the center of the vegetable gratin, when it is soft, it is cooked