In a medium bowl, whisk together the Heavy Cream (3 cup), Salt (2 teaspoon), Freshly Ground Black Pepper (1 teaspoon), Garlic (3 clove), Fresh Thyme (1 tablespoon), Fresh Rosemary (1 teaspoon), Fresh Sage (1 tablespoon), Whole Nutmeg (1 1/4 teaspoon) and Cayenne Pepper (1/4 teaspoon). Set aside.
Mix together the Gruyère Cheese (2 1/2 cup) and Parmigiano-Reggiano (2 cup). Set aside.
Peel Sweet Potato (2 pound) and Russet Potato (1.5 pound). Using a handheld mandolin slicer or a sharp knife, slice potatoes into thin rounds (somewhere between 1/16 and ¼-inch thick slices). Be careful with the mandolin slicer – most common form of severe cuts in the kitchen!
Preheat oven to 425 degrees F (220 degrees C). Spray a 9x13-inch casserole dish with non-stick cooking spray.
Pour ½ cup of the cream mixture on the bottom (this helps keep the potatoes from sticking to the bottom and burning). Layer one layer of sweet potato slices in a circular pattern, overlapping them slightly.
Drizzle ¾ cup of the cream mixture on top, then sprinkle 1 cup of cheese on top. Next layer with white potato slices, drizzle ¾ cup of cream mixture, and sprinkle with 1 cup of cheese on top.
Finish with final layer of sweet potato slices, drizzle with remaining cream mixture, and sprinkle with remaining shredded cheese. Garnish with Fresh Thyme (to taste).
Cover with foil and bake for 15 minutes.
Remove the foil, reduce heat to 400 degrees, and bake until the top is golden and bubbly, 45 to 50 minutes.
Remove from oven and let cool slightly before serving. The sauces and cheese will thicken slightly as it sits.