Preheat the oven to 425 F. Wash and peel the potato(es). Thinly slice the potatoes to 1/8" thick, preferably using a mandoline. Transfer the sliced potatoes to a large bowl of cold water to soak for 10 minutes.
Finely chop the onion (there should be about 1/2 cup), and mince the garlic.
Coat a medium-sized (mine is about 9"x7"x2") ceramic baking dish with about 1/2 tbsp. of unsalted butter. Cut the remaining butter into small cubes, and set aside.
Drain the water from the potatoes, and dry them thoroughly using paper towels or clean kitchen towels. Sprinkle 1/3 of the chopped onions into the bottom of the prepared baking dish, then top the onions with a layer of potato slices, using about half of the potatoes.
Sprinkle the potato layer with a pinch each of salt and pepper, and 1/4 tsp. of thyme. Pour 1/3 cup of heavy cream over the potatoes, then top them with half of the minced garlic and grated parmesan.
Sprinkle another 1/3 of the onions onto the potatoes, then layer the remaining potatoes on top. Sprinkle this layer of potatoes with a pinch each of salt and pepper, then the remaining thyme.
Pour the remaining heavy cream over the top, then sprinkle with the remaining onions. Cover the entire dish with the rest of the parmesan (adding more if you like!) and scatter the butter cubes over the top.
Set the dish on a large, rimmed baking sheet* and cover the dish tightly with foil. Bake for 30 minutes at 425 F. Carefully remove the foil, and bake uncovered for another 25-30 minutes until the potatoes are tender. Broil for 5 minutes for a crunchier, deeply golden top.
Let the potatoes rest for 10 minutes before serving. Enjoy!
*This prevents any liquids from bubbling out of the dish and falling onto the bottom of the oven!
Keyword garlic parmesan scalloped potatoes, garlic scalloped potatoes, scalloped potatoes for two