Position a rack in the upper third of the oven and preheat to 425 degrees F.
Heat an oven safe skillet over medium high heat. Add some (less than half) of the butter and the onions to the pan. Let cook until they start to brown, while stirring only occasionally, about 3-4 minutes. Add the garlic and cook for another minute.
Add half of the potatoes and toss with the onions and garlic to evenly coat, then spread them out. Sprinkle with a hefty pinch of salt, half of the remaining cut up butter, half of the shredded cheese, and pepper to taste.
Arrange the remaining potatoes on top. Sprinkle with another hefty pinch of salt as well as some freshly ground pepper to taste.
Mix the heavy cream and buttermilk together and pour it over the potatoes. Top with nutmeg and allow to simmer for 3 minutes. Dot the potatoes with the remaining cut-up butter.
Mix the remaining shredded cheese with the parmesan and spread this cheese blend over the hot potatoes.
Bake until golden, about 25 minutes. Be sure to check if they are done by tasting a potato, not just by poking it with a fork. Let rest 5 minutes before serving.