Preheat the oven to 400 F and grease the sides of a baking dish with butter or olive oil.

Heat butter in a small skillet over medium high heat, add chopped onion, and saute until tender and translucent, about 2-3 minutes. Add minced garlic, and cook for 30 seconds until fragrant. Pour heavy whipping cream and stir. In a small bowl, make a slurry of the warm heavy cream and arrowroot starch. Add back to the skillet and stir well until sauce thickens. Turn off the heat and set aside.

Place sliced sweet potatoes overlapping each other in one layer in the baking dish. Season with salt, pepper, and rosemary. Pour half of the onion mixture, and sprinkle ½ cup of the parmesan cheese on top.

Place another layer of overlapping sweet potato slices on top. Season them with salt, pepper, and rosemary. Pour remaining onion mixture, and sprinkle remaining Parmesan cheese on top.

Bake in a preheated oven for 30 - 40 minutes until the sweet potatoes are tender and cooked through. Let the sweet potato gratin cool for 5 - 10 minutes, and serve warm. The sauce will thicken as it cools.