Preheat the oven to 350 degrees. Grease the bottom and sides of a 2.5-3 quart baking dish with non-stick cooking spray and set aside.

In a measuring cup mix the heavy cream, garlic, sage, thyme, salt and pepper together well. Set aside.

Place ¼ of the sweet potatoes into the baking dish overlapping each other working clockwise then down the center. Pour ¼ of the cream mix over the sweet potatoes and sprinkle with 2 tablespoons of the Parmesan cheese.

Continue layering with remaining potatoes, cream and cheese to have 4 layers total. On the final layer make sure to spoon out the settled seasoning/garlic from the cream sauce.

Bake uncovered 350 degrees for 40-45 minutes. Using a paring knife, press into the center; if the potatoes give resistance, continue cooking for an additional 15 minutes which will also help brown the top.

Let the gratin sit for about 10 minutes, garnish with more sage and grated parmesan cheese if desired and serve.