Cheesy Scalloped Potatoes

unsalted butter (cut into pieces)

small onion (or half of a medium, minced)

garlic

shredded cheese (I used a cheddar gruyere blend)

russet potatoes (about 3 potatoes, peeled and sliced thin)

Kosher salt and freshly ground pepper

heavy cream

buttermilk

freshly grated nutmeg

grated parmesan cheese