Whisk the cream, milk, salt, pepper, garlic, thyme, rosemary, sage, nutmeg, and cayenne in a big bowl. Set aside.
Peel the sweet potatoes (and Russets if so inclined), then using a food processor, slice the potatoes into thin rounds. If you don't have a food processor, use a mandolin or sharp knife by hand. Combine all slices in a large bowl and toss, mixing the two types of potatoes.
Pour 1/2 cup of the cream on the bottom of a Dutch Oven to prevent the potatoes from sticking to the bottom. Then layer 1/4 of your potato mixture evenly on top.
Drizzle 1/3 of the cream mixture on top, then sprinkle 1/2 cup Parmesan and top with a thin layer of Swiss.
Top with potatoes and repeat for another three layers.
Cover with the Dutch Oven lid or foil and bake until bubbly, 15 minutes. If using foil, be carfeul to not let it contact the top cheese layer.
Remove the lid, reduce the heat to 400°F, and bake until the top is golden and bubbly, 45-50 minutes.
Let cool slightly, then serve hot or at room temperature