Preheat oven to 400°F (unless preparing in advance).
Boil the potatoes: Boil the potatoes for 4 minutes in salted water. Drain and set aside.
Sauté the onions, sprinkle with salt and herbs: Heat the butter over medium-high heat in a large sauté pan and cook the onions, stirring occasionally, until they are softened, about 5 minutes.
Sprinkle them with salt and the herbes de provence. Turn off the heat and set aside.
Layer potatoes, cheeses, onions, nutmeg in baking dish, pour over with half and half: Spread half of the sliced potatoes over the bottom of an 11x7-inch baking dish (Pyrex works well). Sprinkle well with salt and white pepper.
Sprinkle with about a third of the Gruyure and Parmesan cheeses. Top with the onions, the nutmeg and another 1/3 of the cheeses.
Layer the remaining sliced potatoes on top of the cheeses.
Pour the half and half over everything. Sprinkle again with salt and pepper.
Sprinkle the remaining one-third of the cheeses over everything. (At this point you can make a day ahead, just cover and refrigerate until time to cook.)
Bake: Bake uncovered in 400°F oven until cream thickens and everything is nicely browned, about 25-30 minutes.