Add the Whipping Cream (3/4 cup), Half and Half (1 1/4 cup), Garlic (3 clove), and Salt (1/4 teaspoon) in a medium saucepan. Bring the liquid to a boil over medium heat, then decrease the heat to medium-low and simmer for another 20 minutes.
Remove the pan from the heat, and set aside.
Start preheating your oven to 400 degrees F (200 degrees C). Use the Butter (1/4 tablespoon) to generously grease a baking dish that measures 9x13x2-inches (or equivalent).
To assemble the gratin, spoon about 1/4 cup of the cream mixture into the bottom of the prepared baking dish. Place a layer of Sweet Potato (5), slightly overlapping in the bottom of the dish.
Ladle another ¼ cup of the cream over the sweet potatoes, 1/2 Tbsp of Fresh Sage (2 tablespoon), a sprinkle of Dried Thyme (1 tablespoon), Salt and Pepper (to taste) and a tablespoon of Parmesan Cheese (3/4 cup).
Continue the layers until you run out of potatoes. When you get to the last layer, pour the remaining cream over the top, add fresh sage, dried thyme, herbs, salt and pepper, but do not add the final layer of cheese.
Lay a sheet of aluminum foil loosely over the top, and bake covered for about 30-35 minutes.
Remove the foil and bake for another 20 minutes.
Sprinkle the remaining Parmesan over the top of the potatoes and bake for an additional 15 minutes, or until potatoes are cooked.
The gratin is done when a paring knife is easily inserted into the center. Remove from the oven and allow to cool for about 10 minutes before serving.