Preheat the oven to 425°F. Whisk the cream, salt, pepper, garlic, thyme, rosemary, sage, nutmeg, and cayenne in a big bowl. Toss the gruyère and Parm in another bowl.

Peel both kinds of potatoes, then use a handheld mandoline slicer or a sharp knife to slice the potatoes into 1/8-inch-thick rounds. 

Pour ½ cup of the cream mixture on the bottom of a 9-x-13-inch casserole dish, then layer one color of the potatoes in as nice of a shingled pattern as you can, overlapping them slightly. Drizzle ¾ cup of the cream mixture on top, then sprinkle 1 cup of the cheese mixture on top of that. Repeat with another 2 layers of cream, potatoes (alternating layers between types of potatoes), and cheese, then finish with a top layer of potatoes and sprinkle cheese on top of that. Garnish with fresh thyme. 

Cover with foil and bake until bubbly, 30 minutes. Remove the foil, reduce heat to 400°F, and bake until the top is golden and bubbly, 30 minutes. If you want a browner top raise the hest to broil and broil, watching carefully, until the top is a deeper golden brown, 1-2 minutes. Remove from the oven and allow to cool slightly before serving.