Preheat oven to 400°.
Wash and peel sweet potatoes and slice thinly, about 1/4" thick with either a sharp knife or a mandolin.
Butter a baking dish--an oval works beautifully- and arrange the sweet potatoes in a circular fashion beginning at the outside edge to the inside.
Sprinkle the potatoes with salt and pepper.
Add the butter, rosemary, sage and minced garlic to a saucepan to melt and infuse together for about a minute.
Add in the cream to the butter mixture and pour over the sweet potatoes.
Sprinkle with Parmesan cheese.
Cover the top with foil or a lid and bake in a preheated 400° oven for 45 minutes to an hour or until the gratin is done.
OPTIONAL: Remove the cover after 45 minutes and bake until the tops of the sweet potatoes are lightly browned and crisp and the potatoes are tender for about 15-20 more minutes.
Garnish with more sage, if desired, and serve immediately.